New theme elements

[FichteFoll]

I added the new attributes in #210 because they are needed for proper highlighting, but I haven't added the new elements because I'm waiting on an update of the official docs for those.

https://www.sublimetext.com/docs/3/dev/themes.html

I might also need to tweak the CSS color function since ST added a few custom modifier functions that the CSS4 draft does not have. Most likely need to implement my own version of that instead of relying on the shipped CSS.sublime-syntax. Update: fixed by #229

[FichteFoll]

@deathaxe considering that you added many new theme properties, attributes, etc recently, do you happen to know where we stand with out coverage currently?

[deathaxe]

Not for sure. Just added features/fixed stuff I realized in the Default.sublime-them while working on DA UI successor. Did not walk through the latest docs in detail.

[FichteFoll]

Went through the changelog on the docs and added all things still missing in 8411ab6 and 351a4c9.