This commit allows the theme designer to just enable and disable the
effect (enable=true). If no color values are set, values are picked
based on the background color. This essentially restores the hardcoded
values in some other places, and makes them available as sensible
defaults to other themes.
Also, make sure colors are read before the effect is applied (as we need
the new bg color to decide whether it's light or dark).
d9e43ff979