CSS Overview

Overview of CSS in GTK

This chapter describes in detail how GTK uses CSS for styling and layout.

We loosely follow the CSS value definition specification in the formatting of syntax productions.

  • Nonterminals are enclosed in angle backets (〈〉), all other strings that are not listed here are literals
  • Juxtaposition means all components must occur, in the given order
  • A double ampersand (&&) means all components must occur, in any order
  • A double bar (||) means one or more of the components must occur, in any order
  • A single bar (|) indicates an alternative; exactly one of the components must occur
  • Brackets ([]) are used for grouping
  • A question mark (?) means that the preceding component is optional
  • An asterisk (*) means zero or more copies of the preceding component
  • A plus (+) means one or more copies of the preceding component
  • A number in curly braces ({n}) means that the preceding component occurs exactly n times
  • Two numbers in curly braces ({m,n}) mean that the preceding component occurs at least m times and at most n times

CSS nodes

GTK applies the style information found in style sheets by matching the selectors against a tree of nodes. Each node in the tree has a name, a state and possibly style classes. The children of each node are linearly ordered.

Every widget has one or more of these CSS nodes, and determines their name, state, style classes and how they are layed out as children and siblings in the overall node tree. The documentation for each widget explains what CSS nodes it has.

The CSS nodes of a GtkScale

scale[.fine-tune]
├── marks.top
│   ├── mark
┊   ┊
│   ╰── mark
├── trough
│   ├── slider
│   ├── [highlight]
│   ╰── [fill]
╰── marks.bottom
    ├── mark
    ┊
    ╰── mark

Style sheets

The basic structure of the style sheets understood by GTK is a series of statements, which are either rule sets or “@-rules”, separated by whitespace.

A rule set consists of a selector and a declaration block, which is a series of declarations enclosed in curly braces. The declarations are separated by semicolons. Multiple selectors can share the same declaration block, by putting all the separators in front of the block, separated by commas.

A rule set with two selectors

button, entry {
  color: #ff00ea;
  font: 12px "Comic Sans";
}

Importing style sheets

GTK supports the CSS import rule, in order to load another style sheet in addition to the currently parsed one.

The syntax for import rules is as follows:

〈import rule〉 = @import [ 〈url〉 | 〈string〉 ]
〈url〉 = url( 〈string〉 )

An example for using the import rule

@import url("path/to/common.css");

To learn more about the import rule, you can read the Cascading module of the CSS specification.

Selectors

Selectors work very similar to the way they do in CSS.

All widgets have one or more CSS nodes with element names and style classes. When style classes are used in selectors, they have to be prefixed with a period. Widget names can be used in selectors like IDs. When used in a selector, widget names must be prefixed with a # character.

In more complicated situations, selectors can be combined in various ways. To require that a node satisfies several conditions, combine several selectors into one by concatenating them. To only match a node when it occurs inside some other node, write the two selectors after each other, separated by whitespace. To restrict the match to direct children of the parent node, insert a > character between the two selectors.

Theme labels that are descendants of a window

window label {
  background-color: #898989;
}

Theme notebooks, and anything within

notebook {
  background-color: #a939f0;
}

Theme combo boxes, and entries that are direct children of a notebook

combobox,
notebook > entry {
  color: @fg_color;
  background-color: #1209a2;
}

Theme any widget within a GtkBox

box * {
  font: 20px Sans;
}

Theme a label named title-label

label#title-label {
  font: 15px Sans;
}

Theme any widget named main-entry

#main-entry {
  background-color: #f0a810;
}

Theme all widgets with the style class entry

.entry {
  color: #39f1f9;
}

Theme the entry of a GtkSpinButton

spinbutton entry {
  color: #900185;
}

It is possible to select CSS nodes depending on their position amongst their siblings by applying pseudo-classes to the selector, like :first-child, :last-child or :nth-child(even). When used in selectors, pseudo-classes must be prefixed with a : character.

Theme labels in the first notebook tab

notebook tab:first-child label {
  color: #89d012;
}

Another use of pseudo-classes is to match widgets depending on their state. The available pseudo-classes for widget states are :active, :hover :disabled, :selected, :focus, :indeterminate, :checked and :backdrop. In addition, the following pseudo-classes don’t have a direct equivalent as a widget state: :dir(ltr) and :dir(rtl) (for text direction); :link and :visited (for links); :drop(active) (for highlighting drop targets). Widget state pseudo-classes may only apply to the last element in a selector.

Theme pressed buttons

button:active {
  background-color: #0274d9;
}

Theme buttons with the mouse pointer over it

button:hover {
  background-color: #3085a9;
}

Theme insensitive widgets

*:disabled {
  background-color: #320a91;
}

Theme checkbuttons that are checked

checkbutton:checked {
  background-color: #56f9a0;
}

Theme focused labels

label:focus {
  background-color: #b4940f;
}

Theme inconsistent checkbuttons

checkbutton:indeterminate {
  background-color: #20395a;
}

To determine the effective style for a widget, all the matching rule sets are merged. As in CSS, rules apply by specificity, so the rules whose selectors more closely match a node will take precedence over the others.

The full syntax for selectors understood by GTK can be found in the table below. The main difference to CSS is that GTK does not currently support attribute selectors.

Pattern Matches Reference Notes
* any node CSS -
E any node with name E CSS -
E.class any E node with the given style class CSS -
E#id any E node with the given ID CSS GTK uses the widget name as ID
E:nth-child(〈nth-child〉) any E node which is the n-th child of its parent node CSS -
E:nth-last-child(〈nth-child〉) any E node which is the n-th child of its parent node, counting from the end CSS -
E:first-child any E node which is the first child of its parent node CSS -
E:last-child any E node which is the last child of its parent node CSS -
E:only-child any E node which is the only child of its parent node CSS Equivalent to E:first-child:last-child
E:link, E:visited any E node which represents a hyperlink, not yet visited (:link) or already visited (:visited) CSS Corresponds to GTK_STATE_FLAG_LINK and GTK_STATE_FLAGS_VISITED
E:active, E:hover, E:focus any E node which is part of a widget with the corresponding state CSS Corresponds to GTK_STATE_FLAG_ACTIVE, GTK_STATE_FLAG_PRELIGHT and GTK_STATE_FLAGS_FOCUSED; GTK also allows E:prelight and E:focused
E:disabled any E node which is part of a widget which is disabled CSS Corresponds to GTK_STATE_FLAG_INSENSITIVE; GTK also allows E:insensitive
E:checked any E node which is part of a widget (e.g. radio- or checkbuttons) which is checked CSS Corresponds to GTK_STATE_FLAG_CHECKED
E:indeterminate any E node which is part of a widget (e.g. radio- or checkbuttons) which is in an indeterminate state CSS3, CSS4 Corresponds to GTK_STATE_FLAG_INCONSISTENT; GTK also allows E:inconsistent
E:backdrop, E:selected any E node which is part of a widget with the corresponding state - Corresponds to GTK_STATE_FLAG_BACKDROP, GTK_STATE_FLAG_SELECTED
E:not(〈selector〉) any E node which does not match the simple selector 〈selector〉 CSS -
E:dir(ltr), E:dir(rtl) any E node that has the corresponding text direction CSS4 -
E:drop(active) any E node that is an active drop target for a current DND operation CSS4 -
E F any F node which is a descendent of an E node CSS -
E > F any F node which is a child of an E node CSS -
E ~ F any F node which is preceded by an E node CSS -
E + F any F node which is immediately preceded by an E node CSS -

Where:

〈nth-child〉 = even | odd | 〈integer〉 | 〈integer〉n | 〈integer〉n [ + | - ] 〈integer〉

To learn more about selectors in CSS, read the Selectors module of the CSS specification.

Colors

CSS allows to specify colors in various ways, using numeric values or names from a predefined list of colors.

〈color〉 = currentColor | transparent | 〈color name〉 | 〈rgb color〉 | 〈rgba color〉 | 〈hex color〉 | 〈gtk color〉
〈rgb color〉 = rgb( 〈number〉, 〈number〉, 〈number〉 ) | rgb( 〈percentage〉, 〈percentage〉, 〈percentage〉 )
〈rgba color〉 = rgba( 〈number〉, 〈number〉, 〈number〉, 〈alpha value〉 ) | rgba( 〈percentage〉, 〈percentage〉, 〈percentage〉, 〈alpha value〉 )
〈hex color〉 = #〈hex digits〉
〈alpha value〉 = 〈number〉, clamped to values between 0 and 1

The keyword currentColor resolves to the current value of the color property when used in another property, and to the inherited value of the color property when used in the color property itself.

The keyword transparent can be considered a shorthand for rgba(0,0,0,0).

For a list of valid color names and for more background on colors in CSS, see the Color module of the CSS specification.

Specifying colors in various ways

color: transparent;
background-color: red;
border-top-color: rgb(128,57,0);
border-left-color: rgba(10%,20%,30%,0.5);
border-right-color: #ff00cc;
border-bottom-color: #ffff0000cccc;

GTK adds several additional ways to specify colors.

〈gtk color〉 = 〈symbolic color〉 | 〈color expression〉 | 〈win32 color〉

The first is a reference to a color defined via a define`-color` rule. The syntax fordefine-color rules is as follows:

〈define color rule〉 = @define-color 〈name〉 〈color〉

To refer to the color defined by a `define-colorrule, use the name from the rule, prefixed with@`.

〈symbolic color〉 = @〈name〉

An example for defining colors

@define-color bg_color #f9a039;

* {
  background-color: @bg_color;
}

GTK also supports color expressions, which allow colors to be transformed to new ones and can be nested, providing a rich language to define colors. Color expressions resemble functions, taking 1 or more colors and in some cases a number as arguments.

shade() leaves the color unchanged when the number is 1 and transforms it to black or white as the number approaches 0 or 2 respectively. For mix(), 0 or 1 return the unaltered 1st or 2nd color respectively; numbers between 0 and 1 return blends of the two; and numbers below 0 or above 1 intensify the RGB components of the 1st or 2nd color respectively. alpha() takes a number from 0 to 1 and applies that as the opacity of the supplied color.

〈color expression〉 = lighter( 〈color〉 ) | darker( 〈color〉 ) | shade( 〈color〉, 〈number〉 ) |
                     alpha( 〈color〉, 〈number〉 ) | mix( 〈color〉, 〈color〉, 〈number〉 )

On Windows, GTK allows to refer to system colors, as follows:

〈win32 color〉 = -gtk-win32-color( 〈name〉, 〈integer〉 )

Images

CSS allows to specify images in various ways, for backgrounds and borders.

〈image〉 = 〈url〉 | 〈crossfade〉 | 〈alternatives〉 | 〈gradient〉 | 〈gtk image〉
〈crossfade〉 = cross-fade( 〈percentage〉, 〈image〉, 〈image〉 )
〈alternatives〉 = image([ 〈image〉, ]* [ 〈image〉 | 〈color〉 ] )
〈gradient〉 = 〈linear gradient〉 | 〈radial gradient〉
〈linear gradient〉 = [ linear-gradient | repeating-linear-gradient ] (
                      [ [ 〈angle〉 | to 〈side or corner〉 ] , ]?
                      〈color stops〉 )
〈radial gradient〉 = [ radial-gradient | repeating-radial-gradient ] (
                      [ [ 〈shape〉 || 〈size〉 ] [ at 〈position〉 ]? , | at 〈position〉, ]?
                      〈color stops〉 )
〈side or corner〉 = [ left | right ] || [ top | bottom ]
〈color stops〉 =  〈color stop〉 [ , 〈color stop〉 ]+
〈color stop〉 = 〈color〉 [ 〈percentage〉 | 〈length〉 ]?
〈shape〉 = circle | ellipse
〈size〉 = 〈extent keyword〉 | 〈length〉 | [ 〈length〉 | 〈percentage〉 ]{1,2}
〈extent keyword〉 = closest-size | farthest-side | closest-corner | farthest-corner

The simplest way to specify an image in CSS is to load an image file from a URL. CSS does not specify anything about supported file formats; within GTK, you can expect at least PNG, JPEG and SVG to work. The full list of supported image formats is determined by the available gdk-pixbuf image loaders and may vary between systems.

Loading an image file

button {
  background-image: url("water-lily.png");
}

A crossfade lets you specify an image as an intermediate between two images. Crossfades are specified in the draft of the level 4 Image module of the CSS specification.

Crossfading two images

button {
  background-image: cross-fade(50%, url("water-lily.png"), url("buffalo.jpg"));
}

The image() syntax provides a way to specify fallbacks in case an image format may not be supported. Multiple fallback images can be specified, and will be tried in turn until one can be loaded successfully. The last fallback may be a color, which will be rendered as a solid color image.

Image fallback

button {
  background-image: image(url("fancy.svg"), url("plain.png"), green);
}

Gradients are images that smoothly fades from one color to another. CSS provides ways to specify repeating and non-repeating linear and radial gradients. Radial gradients can be circular, or axis-aligned ellipses. In addition to CSS gradients, GTK has its own -gtk-gradient extensions.

A linear gradient is created by specifying a gradient line and then several colors placed along that line. The gradient line may be specified using an angle, or by using direction keywords.

Linear gradients

button {
  background-image: linear-gradient(45deg, yellow, blue);
}

label {
  background-image: linear-gradient(to top right, blue 20%, #f0f 80%);
}

A radial gradient is created by specifying a center point and one or two radii. The radii may be given explicitly as lengths or percentages or indirectly, by keywords that specify how the end circle or ellipsis should be positioned relative to the area it is derawn in.

Radial gradients

button {
  background-image: radial-gradient(ellipse at center, yellow 0%, green 100%);
}

label {
  background-image: radial-gradient(circle farthest-side at left bottom, red, yellow 50px, green);
}

To learn more about gradients in CSS, including details of how color stops are placed on the gradient line and keywords for specifying radial sizes, you can read the Image module of the CSS specification.

GTK extends the CSS syntax for images and also uses it for specifying icons.

〈gtk image〉 = 〈gtk gradient〉 | 〈themed icon〉 | 〈scaled image〉 | 〈recolored image〉 | 〈win32 theme part〉

GTK supports an alternative syntax for linear and radial gradients (which was implemented before CSS gradients were supported).

〈gtk gradient〉 = 〈gtk linear gradient〉 | 〈gtk radial gradient〉
〈gtk linear gradient〉 = -gtk-gradient(linear,
                          [ 〈x position〉 〈y position〉 , ]{2}
                          〈gtk color stops〉 )
〈gtk radial gradient〉 = -gtk-gradient(radial,
                          [ 〈x position〉 〈y position〉 , 〈radius〉 , ]{2}
                          〈gtk color stops〉 )
〈x position〉 = left | right | center | 〈number〉
〈y position〉 = top | bottom | center | 〈number〉
〈radius 〉 = 〈number〉
〈gtk color stops〉 = 〈gtk color stop〉 [ , 〈gtk color stop〉 ]+
〈gtk color stop〉 = color-stop( 〈number〉 , 〈color〉 ) | from( 〈color〉 ) | to( 〈color〉 )

The numbers used to specify x and y positions, radii, as well as the positions of color stops, must be between 0 and 1. The keywords for for x and y positions (left, right, top, bottom, center), map to numeric values of 0, 1 and 0.5 in the obvious way. Color stops using the from() and to() syntax are abbreviations for color-stop with numeric positions of 0 and 1, respectively.

Linear gradients

button {
  background-image: -gtk-gradient (linear,
                                   left top, right bottom,
                                   from(yellow), to(blue));
}
label {
  background-image: -gtk-gradient (linear,
                                   0 0, 0 1,
                                   color-stop(0, yellow),
                                   color-stop(0.2, blue),
                                   color-stop(1, #0f0));
}

Radial gradients

button {
  background-image: -gtk-gradient (radial,
                                   center center, 0,
                                   center center, 1,
                                   from(yellow), to(green));
}
label {
  background-image: -gtk-gradient (radial,
                                   0.4 0.4, 0.1,
                                   0.6 0.6, 0.7,
                                   color-stop(0, #f00),
                                   color-stop(0.1, $a0f),
                                   color-stop(0.2, yellow),
                                   color-stop(1, green));
}

GTK has extensive support for loading icons from icon themes. It is accessible from CSS with the -gtk-icontheme syntax.

〈themed icon〉 = -gtk-icontheme( 〈icon name〉 )

The specified icon name is used to look up a themed icon, while taking into account the values of the -gtk-icon-theme and -gtk-icon-palette properties. This kind of image is mainly used as value of the -gtk-icon-source property.

Using themed icons in CSS

spinner {
  -gtk-icon-source: -gtk-icontheme('process-working-symbolic');
  -gtk-icon-palette: success blue, warning #fc3, error magenta;
}
arrow.fancy {
  -gtk-icon-source: -gtk-icontheme('pan-down');
  -gtk-icon-theme: 'Oxygen';
}

GTK supports scaled rendering on hi-resolution displays. This works best if images can specify normal and hi-resolution variants. From CSS, this can be done with the -gtk-scaled syntax.

〈scaled image〉 = -gtk-scaled( 〈image〉[ , 〈image〉 ]* )

While -gtk-scaled accepts multiple higher-resolution variants, in practice, it will mostly be used to specify a regular image and one variant for scale 2.

Scaled images in CSS

arrow {
  -gtk-icon-source: -gtk-scaled(url('my-arrow.png'),
                                url('my-arrow@2.png'));
}

〈recolored image〉 = -gtk-recolor( 〈url〉 [ , 〈color palette〉 ] )

Symbolic icons from the icon theme are recolored according to the -gtk-icon-palette property. The recoloring is sometimes needed for images that are not part of an icon theme, and the -gtk-recolor syntax makes this available. -gtk-recolor requires a url as first argument. The remaining arguments specify the color palette to use. If the palette is not explicitly specified, the current value of the -gtk-icon-palette property is used.

Recoloring an image

arrow {
  -gtk-icon-source: -gtk-recolor(url('check.svg'), success blue, error rgb(255,0,0));
}

On Windows, GTK allows to refer to system theme parts as images, as follows:

〈win32 theme part〉 = -gtk-win32-theme-part( 〈name〉, 〈integer〉 〈integer〉
                                              [ , [ over( 〈integer〉 〈integer〉 [ , 〈alpha value〉 ]? ) | margins( 〈integer〉{1,4} ) ] ]* )

Transitions

CSS defines a mechanism by which changes in CSS property values can be made to take effect gradually, instead of all at once. GTK supports these transitions as well.

To enable a transition for a property when a rule set takes effect, it needs to be listed in the transition-property property in that rule set. Only animatable properties can be listed in the transition-property.

The details of a transition can modified with the transition-duration, transition-timing-function and transition-delay properties.

To learn more about transitions, you can read the Transitions module of the CSS specification.

Animations

In addition to transitions, which are triggered by changes of the underlying node tree, CSS also supports defined animations. While transitions specify how property values change from one value to a new value, animations explicitly define intermediate property values in keyframes.

Keyframes are defined with an @-rule which contains one or more of rule sets with special selectors. Property declarations for nonanimatable properties are ignored in these rule sets (with the exception of animation properties).

〈keyframe rule〉 = @keyframes 〈name〉 { 〈animation rule〉 }
〈animation rule〉 = 〈animation selector〉 { 〈declaration〉* }
〈animation selector〉 = 〈single animation selector〉 [ , 〈single animation selector〉 ]*
〈single animation selector〉 = from | to | 〈percentage〉

To enable an animation, the name of the keyframes must be set as the value of the animation-name property. The details of the animation can modified with the animation-duration, animation-timing-function, animation-iteration-count and other animation properties.

A CSS animation

@keyframes spin {
  to { -gtk-icon-transform: rotate(1turn); }
}

spinner {
  animation-name: spin;
  animation-duration: 1s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
}

To learn more about animations, you can read the Animations module of the CSS specification.

Key bindings

In order to extend key bindings affecting different widgets, GTK supports the binding-set rule to parse a set of bind/unbind directives. Note that in order to take effect, the binding sets defined in this way must be associated with rule sets by setting the -gtk-key-bindings property.

The syntax for binding-set rules is as follows:

〈binding set rule〉 = @binding-set 〈binding name〉 { [ [ 〈binding〉 | 〈unbinding〉 ] ; ]* }
〈binding〉 = bind "〈accelerator〉" { 〈signal emission〉* }
〈signal emission〉 = "〈signal name〉" ( [ 〈argument〉 [ , 〈argument〉 ]* ]? }
〈unbinding〉 = unbind "〈accelerator〉"

where 〈accelerator〉 is a string that can be parsed by gtk_accelerator_parse(), 〈signal name〉 is the name of a keybinding signal of the widget in question, and the 〈argument〉 list must be according to the signals declaration.

An example for using the binding-set rule

@binding-set binding-set1 {
  bind "<alt>Left" { "move-cursor" (visual-positions, -3, 0) };
  unbind "End";
};

@binding-set binding-set2 {
  bind "<alt>Right" { "move-cursor" (visual-positions, 3, 0) };
  bind "<alt>KP_space" { "delete-from-cursor" (whitespace, 1)
                         "insert-at-cursor" (" ") };
};

entry {
  -gtk-key-bindings: binding-set1, binding-set2;
}