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

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https://developer.gnome.org/gtk4/4.0/ch39s02.html