## 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