Add mode colors (color-modes) to the "onedark" theme

[usagi-flow]

After the following themes:

autumn
autumn_night
meliora
night_owl
rose_pine
solarized_dark
solarized_light

...comes onedark!

Regarding the colors (blue for normal, green for insert and purple for selection), I took inspiration from lualine, which uses these colors by default.
Additionally, I find it especially intuitive to have green for insertion (which we know from diffs) and blue for the regular, default state.

I opted for a dark foreground, as a light foreground simply lacks any minimal amount of contrast. A very dark gray ("light-black") seemed to yield the best results.

For those who didn't know yet: This wonderful coloring feature comes from #2676 and must be enabled in the configuration by setting color-modes = true under [editor].

[archseer]

Thanks!