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

Percentage

%

Angle

deg, rad, grad, turn

Time

s, ms

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.

Length values using physical units (pt, pc, in, cm, mm) are translated to px using the dpi value specified by the -gtk-dpi property, which is different from the CSS definition, which uses a fixed dpi of 96.

The calc() notation adds considerable expressive power to all of these datatypes. 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 CSS 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

Custom Properties

GTK supports custom properties as defined in the CSS Custom Properties for Cascading Variables spec.

Custom properties are defined as follows:

--prop: red;

and used via the var keyword:

color: var(--prop);

Custom properties can have a fallback for when the referred property is invalid:

color: var(--prop, green);

Colors

CSS Colors

Colors can be expressed in numerous ways in CSS (see the Color Module. GTK supports many (but not all) of these.

You can use rgb(), rgba(), hsl() with both the legacy or the modern CSS syntax, and calc() can be used as well in color expressions. hwb(), oklab(), oklch(), color(), color-mix() and relative colors are supported as well.

Non-CSS Colors

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

These extensions are deprecated and should be replaced by the equivalent standard CSS notions.

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 @.

The standard CSS mechanisms that should be used instead of define-color are custom properties, :root and var().

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 brighter 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)

multiplies the alpha value of color by 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