Add other colors to the stylesheet

[mfbehrens]

I would like to color my boxes yellow. However, yellow is not defined? Is there any good reason, why not all colors should be included?

I can create a pull request if wanted.

[Sebagabones]

Heya! I don’t know why the rest of the colours weren’t defined, I wasn’t the maintainer at the time - however I’m happy for them to be added (and it would make much more sense for them to all be defined)! If you would like to make a PR that would be great (just note you’ll be wanting to change vimium.tera) - otherwise I’m happy to try do it later today :)