Do not hardcode colors for popups and error pane

[nanoant]

Currently OmniSharp-Sublime uses hard-coded colors, which may work for these using dark color styles, but looks bad when using light.

It would be much better if CSS would use pre-defined variables for background, etc. see this change in some other Sublime Text plugin addressing similar issue: https://github.com/DamnWidget/anaconda/commit/ab2fad575b74bd7afe4bf270f106f85d24bc74e3

[Rosalie241]

If you want this implemented, open a PR.