Drop `color()`, `theme-color()` & `gray()` functions (#29083)

Drop `color()`, `theme-color()` & `gray()` functions  (#29083)

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.