Danrabbit/custom colors

[danirabbit]

Todos:

• convert themes to a hashtable and get rid of the theme struct. This will make lookups simpler
• Persist the custom theme values in gsettings
• Avoid a lot of extra work combining and splitting strings
• Save theme name in gsettings so we're not relying on looking up the palette and trying to make a match
• Recalculate contrast only when background and foreground values change

[jeremypw]

@danrabbit Is this something you would like to push ahead with? If so it would make other PRs like #633 redundant.

[jeremypw]

This would probably also affect PRs #546 and #630.

[danirabbit]

Yeah if someone else wants to pick this up, feel free! I just haven't been able to revisit it for a while.

Since this is a pretty big branch, it probably makes sense to try to break it up into smaller cleanups that can be merged before adding full color scheme support

[jeremypw]

@danrabbit Can this PR be closed now?

[danirabbit]

Yes definitely