GTK supports CSS properties and shorthands as far as they can be applied in the context of widgets, and adds its own properties only when needed. All GTK-specific properties have a -gtk prefix.

Basic types

All properties support the following keywords: inherit, initial, unset, with the same meaning as defined in the CSS Cascading and Inheritance spec.

The following units are supported for basic datatypes:

Length
px, pt, em, ex, rem, pc, in, cm, mm, calc()
Percentage
%, calc()
Angle
deg, grad, turn, calc()
Time
s, ms, calc()

Length values with the em or ex units are resolved using the font size value, unless they occur in setting the font-size itself, in which case they are resolved using the inherited font size value.

The rem unit is resolved using the initial font size value, which is not quite the same as the CSS definition of rem.

The calc() notation adds considerable expressive power. There are limits on what types can be combined in such an expression (e.g. it does not make sense to add a number and a time). For the full details, see the CSS3 Values and Units spec.

A common pattern among shorthand properties (called ‘four sides’) is one where one to four values can be specified, to determine a value for each side of an area. In this case, the specified values are interpreted as follows:

4 values:
top right bottom left
3 values:
top horizontal bottom
2 values:
vertical horizontal
1 value:
all

Colors

GTK extends the CSS syntax with several additional ways to specify colors.

The first is a reference to a color defined via a define-color rule in CSS. The syntax for define-color rules is as follows:

`define`-color Name Color

To refer to the color defined by a define-color rule, prefix the name with @.

GTK also supports color expressions, which allow colors to be transformed to new ones. Color expressions 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.

lighter(Color)
produces a brigher variant of Color
darker(Color)
produces a darker variant of Color
shade(Color, Number)
changes the lightness of Color. The number ranges from 0 for black to 2 for white.
alpha(Color, Number)
replaces the alpha value of color with number (between 0 and 1)
mix(Color1, Color2, Number)
interpolates between the two colors

Images

GTK extends the CSS syntax for images and also uses it for specifying icons. To load a themed icon, use

-gtk-icontheme(Name)

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

Symbolic icons from the icon theme are recolored according to the -gtk-icon-palette property, which defines a list of named colors. The recognized names for colors in symbolic icons are error, warning and success. The default palette maps these three names to symbolic colors with the names error_color, warning_color and success_color respectively. The syntax for defining a custom palette is a comma-separated list of name-color pairs, e.g.

success blue, warning #fc3, error magenta

Recoloring is sometimes needed for images that are not part of an icon theme, and the

-gtk-recolor(uri, palette)

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.

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

-gtk-scaled(Image1, Image2)

GTK CSS Properties

Property Reference Notes
color CSS Color Level 3
opacity CSS Color Level 3
filter CSS Filter Effect Level 1
font-family CSS Fonts Level 3 defaults to gtk-font-name setting
font-size CSS Fonts Level 3 defaults to gtk-font-name setting
font-style CSS Fonts Level 3
font-variant CSS Fonts Level 3 only CSS2 values supported
font-weight CSS Fonts Level 3
font-stretch CSS Fonts Level 3
font-kerning CSS Fonts Level 3
font-variant-ligatures CSS Fonts Level 3
font-variant-position CSS Fonts Level 3
font-variant-caps CSS Fonts Level 3
font-variant-numeric CSS Fonts Level 3
font-variant-alternates CSS Fonts Level 3
font-variant-east-asian CSS Fonts Level 3
font-feature-settings CSS Fonts Level 3
font-variation-settings CSS Fonts Level 4
-gtk-dpi Number defaults to screen resolution
font CSS Fonts Level 3 CSS allows line-height, etc
font-variant CSS Fonts Level 3
caret-color CSS Basic User Interface Level 3 CSS allows an auto value
-gtk-secondary-caret-color Color used for the secondary caret in bidirectional text
letter-spacing CSS Text Level 3
text-decoration-line CSS Text Decoration Level 3
text-decoration-color CSS Text Decoration Level 3
text-decoration-style CSS Text Decoration Level 3 CSS allows dashed and dotted
text-shadow CSS Text Decoration Level 3
text-decoration CSS Text Decoration Level 3
-gtk-icon-source Image, builtin or none used for builtin icons in buttons and expanders
-gtk-icon-size Length size used for builtin icons in buttons and expanders
-gtk-icon-style requested, regular or symbolic preferred style for application-loaded icons
-gtk-icon-transform Transform list or none applied to builtin and application-loaded icons
-gtk-icon-palette Color palette, as explained above used to recolor symbolic icons
-gtk-icon-shadow Shadow or none applied to builtin and application-loaded icons
-gtk-icon-filter Filter value list or none applied to builtin and application-loaded icons
transform CSS Transforms Level 1
transform-origin CSS Transforms Level 1 CSS allows specifying a z component
min-width CSS Box Model Level 3 CSS allows percentages
min-height CSS Box Model Level 3 CSS allows percentages
margin-top CSS Box Model Level 3 CSS allows percentages or auto
margin-right CSS Box Model Level 3 CSS allows percentages or auto
margin-bottom CSS Box Model Level 3 CSS allows percentages or auto
margin-left CSS Box Model Level 3 CSS allows percentages or auto
padding-top CSS Box Model Level 3 CSS allows percentages
padding-right CSS Box Model Level 3 CSS allows percentages
padding-bottom CSS Box Model Level 3 CSS allows percentages
padding-left CSS Box Model Level 3 CSS allows percentages
margin CSS Box Model Level 3 a ‘four sides’ property
padding CSS Box Model Level 3 a ‘four sides’ property
border-top-width CSS Backgrounds and Borders Level 3 CSS allows other values
border-right-width CSS Backgrounds and Borders Level 3 CSS allows other values
border-bottom-width CSS Backgrounds and Borders Level 3 CSS allows other values
border-left-width CSS Backgrounds and Borders Level 3 CSS allows other values
border-top-style CSS Backgrounds and Borders Level 3
border-right-style CSS Backgrounds and Borders Level 3
border-bottom-style CSS Backgrounds and Borders Level 3
border-left-style CSS Backgrounds and Borders Level 3
border-top-right-radius CSS Backgrounds and Borders Level 3
border-bottom-right-radius CSS Backgrounds and Borders Level 3
border-bottom-left-radius CSS Backgrounds and Borders Level 3
border-top-left-radius CSS Backgrounds and Borders Level 3
border-top-color CSS Backgrounds and Borders Level 3
border-right-color CSS Backgrounds and Borders Level 3
border-bottom-color CSS Backgrounds and Borders Level 3
border-left-color CSS Backgrounds and Borders Level 3
border-image-source CSS Backgrounds and Borders Level 3
border-image-repeat CSS Backgrounds and Borders Level 3
border-image-slice CSS Backgrounds and Borders Level 3 a ‘four sides’ property
border-image-width CSS Backgrounds and Borders Level 3 a ‘four sides’ property
border-width CSS Backgrounds and Borders Level 3 a ‘four sides’ property
border-style CSS Backgrounds and Borders Level 3 a ‘four sides’ property
border-color CSS Backgrounds and Borders Level 3 a ‘four sides’ property
border-top CSS Backgrounds and Borders Level 3
border-right CSS Backgrounds and Borders Level 3
border-bottom CSS Backgrounds and Borders Level 3
border-left CSS Backgrounds and Borders Level 3
border CSS Backgrounds and Borders Level 3
border-radius CSS Backgrounds and Borders Level 3
border-image CSS Backgrounds and Borders Level 3
outline-style CSS Basic User Interface Level 3 initial value is none, auto is not supported
outline-width CSS Basic User Interface Level 3
outline-color CSS Basic User Interface Level 3 initial value is currentColor, invert is not supported
outline-offset CSS Basic User Interface Level 3
outline CSS Basic User Interface Level 3
background-color CSS Backgrounds and Borders Level 3
background-clip CSS Backgrounds and Borders Level 3
background-origin CSS Backgrounds and Borders Level 3
background-size CSS Backgrounds and Borders Level 3
background-position CSS Backgrounds and Borders Level 3
background-repeat CSS Backgrounds and Borders Level 3
background-image CSS Backgrounds and Borders Level 3 not supported: urls without quotes
box-shadow CSS Backgrounds and Borders Level 3
background-blend-mode CSS Compositing and Blending Level 1 only affects multiple backgrounds
background CSS Backgrounds and Borders Level 3
transition-property CSS Transitions
transition-duration CSS Transitions
transition-timing-function CSS Transitions
transition-delay CSS Transitions
transition CSS Transitions
animation-name CSS Animations Level 1
animation-duration CSS Animations Level 1
animation-timing-function CSS Animations Level 1
animation-iteration-count CSS Animations Level 1
animation-direction CSS Animations Level 1
animation-play-state CSS Animations Level 1
animation-delay CSS Animations Level 1
animation-fill-mode CSS Animations Level 1
animation CSS Animations Level 1
border-spacing CSS Table Level 3 respected by GtkBoxLayout, GtkGridLayout, GtkCenterLayout