Drop `color()`, `theme-color()` & `gray()` functions in favor of variables. The functions just called a `map-get()` of a map where just the variables were defined.
Also the `theme-color-level()` now accepts any color you want instead of only `$theme-colors` colors. The first value now is a variable instead of the `$theme-colors` key.
Media queries do not support CSS custom properties (CSS variables), so we've had some useless variables lying around since shipping v4. This finally removes them and the associated documentation content that explained the situation.
Fixes#29012.
