CSS theme changes

[shkirando]

changelog: Update the UI color schemes (dark, light, contrast)

Example:

[oskarhane]

Haven't looked at the actual visuals yet, just commenting on the code so far, and you need to remove a few comments.

[oskarhane]

This is a fantastic change, the revised dark theme is beautiful!

Just a few things I found that could be improved.

Eye icon:

Row separator is hard to see.

Statusbar left padding on some frames (same with :sysinfo) + code/pre field.

I think code blocks should be colored in some way to distinguish them from regular text. Goes for both light and dark theme.
And the overflow on top of statusbar looks strange.

And btw, can you add a few screenshots to the PR description as well, for others to see, that'd be great.

[oskarhane]

I pushed some small changes (fix failing tests, relationship text color, removal of comments.
This looks very good, thanks for the hard work on this!

Feel free to merge when the tests have gone green.