Also remove the color values from the ComputedStyle object and get them
via StyleProperties instead.
At the moment, we only handle colors that Color::from_string() parses.
This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
