Title: GTK CSS Properties
Slug: css-properties

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](https://www.w3.org/TR/css3-cascade/#defaulting-keywords)
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](https://www.w3.org/TR/css-values-4/) 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

Starting with 4.16, GTK supports custom properties as defined in the
[CSS Custom Properties for Cascading Variables](https://www.w3.org/TR/css-variables-1)
spec.

Custom properties are defined as follows:

```css
--prop: red;
```

and used via the `var` keyword:

```css
color: var(--prop);
```

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

```css
color: var(--prop, green);
```

Older GTK builds (e. g., 4.14) do not support that. As for color variables, you have to use [non-CSS color expressions](#non-css-colors). As for other variable types, there is not direct replacement.

## Colors

### CSS Colors

Colors can be expressed in numerous ways in CSS (see the
[Color Module](https://www.w3.org/TR/css-color-5/). 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 @.

There are standard CSS mechanisms that should be used instead of `define-color`: [custom properties](#custom-properties), `:root` selector and `var()` expression. GTK supports them since 4.16 release.

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

| Property   | Reference | Notes |
|:-----------|:----------|:------|
|color       | [CSS Color Level 3](https://www.w3.org/TR/css3-color/#foreground) | |
|opacity     | [CSS Color Level 3](https://www.w3.org/TR/css3-color/#opacity) | |
|filter      | [CSS Filter Effect Level 1](https://www.w3.org/TR/filter-effects-1/#typedef-filter-value-list) | |
|font-family | [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-family-prop) | defaults to gtk-font-name setting |
|font-size   | [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-size-prop) | defaults to gtk-font-name setting |
|font-style  | [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-style-prop) | |
|font-variant| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#descdef-font-variant) | only CSS2 values supported |
|font-weight | [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-weight-prop) | |
|font-stretch| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-stretch-prop) | |
|font-kerning| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-kerning-prop) | |
|font-variant-ligatures| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-variant-ligatures-prop) | |
|font-variant-position| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-variant-position-prop) | |
|font-variant-caps| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-variant-position-prop) | |
|font-variant-numeric| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-variant-numeric-prop) | |
|font-variant-alternates| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-variant-alternates-prop) | |
|font-variant-east-asian| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-variant-east-asian-prop) | |
|font-feature-settings| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-feature-settings-prop) | |
|font-variation-settings| [CSS Fonts Level 4](https://www.w3.org/TR/css-fonts-4/#font-variation-settings-def) | |
|-gtk-dpi|[Number](https://www.w3.org/TR/css3-values/#number-value) | defaults to screen resolution |
|font| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-prop) | CSS allows line-height, etc |
|font-variant| [CSS Fonts Level 3](https://www.w3.org/TR/css3-fonts/#font-variant-prop) | |
|caret-color|[CSS Basic User Interface Level 3](https://www.w3.org/TR/css3-ui/#caret-color) | CSS allows an auto value |
|-gtk-secondary-caret-color|[Color](https://www.w3.org/TR/css-color-3/#valuea-def-color) | used for the secondary caret in bidirectional text |
|letter-spacing| [CSS Text Level 3](https://www.w3.org/TR/css3-text/#letter-spacing) | |
|text-transform| [CSS Text Level 3](https://www.w3.org/TR/css-text-3/#text-transform-property) | CSS allows full-width and full-size-kana. Since 4.6 |
|line-height| [CSS Inline Layout Level 3](https://www.w3.org/TR/css-inline-3/#line-height-property) | Since 4.6 |
|text-decoration-line| [CSS Text Decoration Level 3](https://www.w3.org/TR/css-text-decor-3/#text-decoration-line-property) | |
|text-decoration-color| [CSS Text Decoration Level 3](https://www.w3.org/TR/css-text-decor-3/#text-decoration-color-property) | |
|text-decoration-style| [CSS Text Decoration Level 3](https://www.w3.org/TR/css-text-decor-3/#text-decoration-style-property) | CSS allows dashed and dotted |
|text-shadow| [CSS Text Decoration Level 3](https://www.w3.org/TR/css-text-decor-3/#text-shadow-property) | |
|text-decoration| [CSS Text Decoration Level 3](https://www.w3.org/TR/css-text-decor-3/#text-decoration-property) | |
|-gtk-icon-source| [Image](https://www.w3.org/TR/css-backgrounds-3/#typedef-image), `builtin` or `none` | used for builtin icons in buttons and expanders |
|-gtk-icon-size| [Length](https://www.w3.org/TR/css3-values/#length-value) | 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](https://www.w3.org/TR/css-transforms-1/#typedef-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](https://www.w3.org/TR/css-backgrounds-3/#typedef-shadow) or `none` | applied to builtin and application-loaded icons |
|-gtk-icon-filter| [Filter value list](https://www.w3.org/TR/filter-effects-1/#typedef-filter-value-list) or `none` | applied to builtin and application-loaded icons |
|-gtk-icon-weight | Like [font weight](https://www.w3.org/TR/css3-fonts/#font-weight-prop) | applied to builtin and application-loaded, stroked symbolic icons |
|transform| [CSS Transforms Level 1](https://www.w3.org/TR/css-transforms-1/#transform-property) | |
|transform-origin| [CSS Transforms Level 1](https://www.w3.org/TR/css-transforms-1/#transform-origin-property) | CSS allows specifying a z component|
|min-width| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#min-width) | CSS allows percentages |
|min-height| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#min-height) | CSS allows percentages |
|margin-top| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#margin-top) | CSS allows percentages or auto |
|margin-right| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#margin-right) | CSS allows percentages or auto |
|margin-bottom| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#margin-bottom) | CSS allows percentages or auto |
|margin-left| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#margin-left) | CSS allows percentages or auto |
|padding-top| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#padding-top) | CSS allows percentages |
|padding-right| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#padding-right) | CSS allows percentages |
|padding-bottom| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#padding-bottom) | CSS allows percentages |
|padding-left| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#padding-left) | CSS allows percentages |
|margin| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#margin) | a 'four sides' property |
|padding| [CSS Box Model Level 3](https://www.w3.org/TR/css3-box/#padding) | a 'four sides' property |
|border-top-width| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-width) | CSS allows other values |
|border-right-width| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-width) | CSS allows other values |
|border-bottom-width| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-width) | CSS allows other values |
|border-left-width| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-width) | CSS allows other values |
|border-top-style| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-style) | |
|border-right-style| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-style) | |
|border-bottom-style| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-style) | |
|border-left-style| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-style) | |
|border-top-right-radius| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-radius) | |
|border-bottom-right-radius| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-radius) | |
|border-bottom-left-radius| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-radius) | |
|border-top-left-radius| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-radius) | |
|border-top-color| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-color) | |
|border-right-color| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-color) | |
|border-bottom-color| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-color) | |
|border-left-color| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-color) | |
|border-image-source| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-image-source) | |
|border-image-repeat| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-image-repeat) | |
|border-image-slice| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-image-slice) | a 'four sides' property |
|border-image-width| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-image-width) | a 'four sides' property |
|border-width| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-width) | a 'four sides' property |
|border-style| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#the-border-style) | a 'four sides' property |
|border-color| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border-color) | a 'four sides' property |
|border-top| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border-top) | |
|border-right| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border-right) | |
|border-bottom| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border-bottom) | |
|border-left| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border-left) | |
|border| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border) | |
|border-radius| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border-radius) | |
|border-image| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#border-image) | |
|outline-style| [CSS Basic User Interface Level 3](https://www.w3.org/TR/css3-ui/#outline-style) | initial value is none, auto is not supported |
|outline-width| [CSS Basic User Interface Level 3](https://www.w3.org/TR/css3-ui/#outline-width) | |
|outline-color| [CSS Basic User Interface Level 3](https://www.w3.org/TR/css3-ui/#outline-color) | initial value is currentColor, invert is not supported |
|outline-offset| [CSS Basic User Interface Level 3](https://www.w3.org/TR/css3-ui/#outline-offset) | |
|outline| [CSS Basic User Interface Level 3](https://www.w3.org/TR/css3-ui/#propdef-outline) | |
|background-color| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background-color) | |
|background-clip| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background-clip) | |
|background-origin| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background-origin) | |
|background-size| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background-size) | |
|background-position| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background-position) | |
|background-repeat| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background-repeat) | |
|background-image| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background-image) | not supported: urls without quotes |
|box-shadow| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#box-shadow) | |
|background-blend-mode| [CSS Compositing and Blending Level 1](https://www.w3.org/TR/compositing-1/#propdef-background-blend-mode) | only affects multiple backgrounds |
|background| [CSS Backgrounds and Borders Level 3](https://www.w3.org/TR/css3-background/#background) | |
|transition-property| [CSS Transitions](https://www.w3.org/TR/css3-transitions/#transition-property) | |
|transition-duration| [CSS Transitions](https://www.w3.org/TR/css3-transitions/#transition-duration) | |
|transition-timing-function| [CSS Transitions](https://www.w3.org/TR/css3-transitions/#transition-timing-function) | |
|transition-delay| [CSS Transitions](https://www.w3.org/TR/css3-transitions/#transition-delay) | |
|transition| [CSS Transitions](https://www.w3.org/TR/css3-transitions/#transition) | |
|animation-name| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-name) | |
|animation-duration| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-duration) | |
|animation-timing-function| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-timing-function) | |
|animation-iteration-count| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-iteration-count) | |
|animation-direction| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-direction) | |
|animation-play-state| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-play-state) | |
|animation-delay| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-delay) | |
|animation-fill-mode| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation-fill-mode) | |
|animation| [CSS Animations Level 1](https://www.w3.org/TR/css3-animations/#animation) | |
|border-spacing| [CSS Table Level 3](https://www.w3.org/TR/css-tables-3/#border-spacing-property) | respected by GtkBoxLayout, GtkGridLayout, GtkCenterLayout |

## Media Queries

Since GTK 4.20 [CSS Media Queries](https://www.w3.org/TR/mediaqueries-5/) are supported.
Media queries can include `not`, `and`, and `or` expressions.

GTK supports the following media features:

| Media feature          | Reference | Notes |
|:-----------------------|:----------|:------|
| prefers-color-scheme   | [Media Queries Level 5](https://www.w3.org/TR/mediaqueries-5/#prefers-color-scheme) | `light` and `dark` mode |
| prefers-contrast       | [Media Queries Level 5](https://www.w3.org/TR/mediaqueries-5/#prefers-contrast) | `no-preference`, `more`, and `less` |
| prefers-reduced-motion | [Media Queries Level 5](https://www.w3.org/TR/mediaqueries-5/#prefers-reduced-motion) | `no-preference`, `reduced` |
