Drop unused CSS variables

[laymonage]

I'm just copying the variables from @primer/primitives and GitHub's website. There are a lot of unused variables in the current theme files. It would be nice if we could remove them at build time. I tried using PurgeCSS but I couldn't find the right configuration that plays well with the project's setup.

If the only option is to trim down the theme files, I'm also okay with it, as long as it's easy enough to re-add any variables that we need in the future.