## GTK CSS Overview GTK CSS Overview — The language used for style information in GTK |

## CSS in GTK

This chapter describes how GTK uses CSS for styling and layout. It is not meant to be an explanation of CSS from first principles, but focuses on listing supported CSS features and differences between Web CSS and GTK.

There is plenty of introductory documentation available that can be used to learn about CSS in general. In the tables below we include links to the official specs that can be used to look up the definition of individual selectors and properties.

### 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.

Example 5. The CSS nodes of a GtkScale

    1
    2
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    4
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    8
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    scale[.fine-tune]
    ├── marks.top
    │   ├── mark
    ┊   ┊
    │   ╰── mark
    ├── trough
    │   ├── slider
    │   ├── [highlight]
    │   ╰── [fill]
    ╰── marks.bottom
        ├── mark
        ┊
        ╰── mark

### Selectors

Selectors work very similar to the way they do on the web.

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.

Table 1. GTK CSS Selectors

Pattern | Reference | Notes
--- | --- | ---
* | CSS Selectors Level 3 |
E | CSS Selectors Level 3 |
E.class | CSS Selectors Level 3 |
E#id | CSS Selectors Level 3 | GTK uses the widget name as ID
E:nth-child(nth-child) | CSS Selectors Level 3 |
E:nth-last-child(nth-child) | CSS Selectors Level 3 |
E:first-child | CSS Selectors Level 3 |
E:last-child | CSS Selectors Level 3 |
E:only-child | CSS Selectors Level 3 | Equivalent to E:first-child:last-child
E:link, E:visited | CSS Selectors Level 3 | Corresponds to GTK_STATE_FLAG_LINK and GTK_STATE_FLAGS_VISITED
E:active, E:hover, E:focus | CSS Selectors Level 3 | Correspond to GTK_STATE_FLAG_ACTIVE, GTK_STATE_FLAG_PRELIGHT, GTK_STATE_FLAGS_FOCUSED
E:focus-within | CSS Selectors Level 4 | Set on all ancestors of the focus widget, unlike CSS
E:focus-visible | CSS Selectors Level 4 | Set on focus widget and all ancestors, unlike CSS
E:disabled | CSS Selectors Level 3 | Corresponds to GTK_STATE_FLAG_INSENSITIVE
E:checked | CSS Selectors Level 3 | Corresponds to GTK_STATE_FLAG_CHECKED
E:indeterminate | CSS3 Selectors Level 3 | Corresponds to GTK_STATE_FLAG_INCONSISTENT
E:backdrop, E:selected |  | Corresponds to GTK_STATE_FLAG_BACKDROP, GTK_STATE_FLAG_SELECTED
E:not(selector) | CSS Selectors Level 3 |
E:dir(ltr), E:dir(rtl) | CSS Selectors Level 4 |
E:drop(active) | CSS Selectors Level 4 |
E F | CSS Selectors Level 3 |
E > F | CSS Selectors Level 3 |
E ~ F | CSS Selectors Level 3 |
E + F | CSS Selectors Level 3 |

## GTK 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 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`)`

Table 2. GTK CSS Properties

Color and Filter Properties
Name | Reference | Notes
--- | --- | ---
color | CSS Color Level 3 |
opacity | CSS Color Level 3 |
filter | CSS Filter Effects Level 1 |
Font Properties
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 Shorthands
font | CSS Fonts Level 3 | CSS allows line-height, etc
font-variant | CSS Fonts Level 3 |
Text Caret Properties
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
Text Decoration Properties
letter-spacing | CSS Text Level 3 |
text-decoration-line | CSS Text Decoration Level 3 | CSS allows overline
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 Shorthands
text-decoration | CSS Text Decoration Level 3 |
Icon Properties
-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
Box Properties
transform | CSS Transforms Level 2 |
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
Box Shorthands
margin | CSS Box Model Level 3 | a 'four sides' property
padding | CSS Box Model Level 3 | a 'four sides' property
Border Properties
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 Shorthands
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 Properties
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 Shorthands
outline | CSS Basic User Interface Level 3 |
Background Properties
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, colors in crossfades
box-shadow | CSS Backgrounds and Borders Level 3 |
background-blend-mode | CSS Compositing and Blending Level 1 | only affects multiple backgrounds
Background Shorthands
background | CSS Backgrounds and Borders Level 3 |
Transition Properties
transition-property | CSS Transitions |
transition-duration | CSS Transitions |
transition-timing-function | CSS Transitions |
transition-delay | CSS Transitions |
Transition Shorthands
transition | CSS Transitions |
Animation Properties
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 Shorthands
animation | CSS Animations Level 1 |
Table-related Properties
border-spacing | CSS Table Level 3 | respected by GtkBox and GtkGrid