fix: make day diff highlights more readable

[riley-martine]

Day diff highlights for delta are hard to read. See before and after:

Very willing to adjust colors, if these are not as desired. (The blue and green might clash a bit?) I fiddled a bit, but had trouble finding something that's distinct from the other background color, but doesn't have significantly worse contrast with the text than that bg color.

(Working on a fix for the _style_name and other bits being hard to read against blue -- these are set in the sublime colors to fixed values regardless of theme. This gets used for the bat colors which delta uses for syntax highlighting. I'm trying to get something that preserves mostly how the dark themes look, but makes day easier to read. Don't want to mess up anyone's existing theme if they like it, yknow.)

[folke]

The problem was actually the colors. Delta. Colors shouldn't be defined in an extra since for the day theme, the colors are automatically generated.

I moved the color defs to colors.lua, so that should now be correct.

Feel free to open another PR with changes to the sublime scheme!