Is there a Theme API or guide?

I’ve searched around a bit, but haven’t found a good list of what elements can be modified. I can, of course, reference the base theme and other themes, but not all themes change all things.

In particular, I’ve been trying to up-res @2dchaos’s theme so that I can use it on my 4k monitors, and there are various elements that end up with poor spacing, and I haven’t been able to find the xml tags that need changing.

If there’s a guide that lists all of the possible things, that’d be really helpful.