Add support for light background

[glensc]

Seems this is designed for dark background, as with light background the filenames are quite hard to read.

[rpdelaney]

That would require maintaining two lists and rebuilding a color scheme from scratch. I have a hard enough time keeping the groups consistent as-is. Maintainers don't use light theme, so I don't see this happening before a major restructuring, e.g. a migration to vivid