GTK+ CSS Overview

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.

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.

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〉 )

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.

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.

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) and :drop(active) (for highlighting drop targets). Widget state pseudo-classes may only apply to the last element in a selector.

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.

〈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〉

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.

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 for @define-color rules is as follows:

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

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

〈symbolic color〉 = @〈name〉

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.

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.

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.

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.

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.

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.

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.

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.

〈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.

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.

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.

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