Possible values are "black20", "black40", "black60", "black80", and
"invert".
The blackXX ones add a black shade on the map (making them look darker,
like unlit caves), that has a 20% to 80% opacity. The invert one make
the screen use inverted colors. Others could be added, but only static
ones, like a B&W mode for example.
Animated ones ( like continuous gamma rotation ) aren't viable
performance-wise.