Feat/additional themes

[tarnung]

#539

[tarnung]

code light and smyck light are not usable right now. I have to tune highlighting and maybe some colors

[tarnung]

now we're good.

[munen]

An initial visual check of the themes shows that they all look amazing! 💯

There is a little bug if there's no theme selected, yet:

The danger of checking against undefined continues(;

Of course, we can fix the issue inline in this PR. But my hunch in #534 (comment) that the lack of a default colorScheme will cause more problems later on is confirmed. So, it would be best if both (default and a more robust check) can be tackled.

[munen]

I looked into it a bit more. It was a good hunch at #534 (comment), indeed. loadTheme is called with the actual arguments "null" (as a string), because there's no default.

[tarnung]

I thought I had covered all the bases by checking against undefined AND "undefined" (as a string).
But actually having a default value would be better, I agree. I'll look into it later this week.

Something else that just came to mind:
I left the colors.css file intact to give the variables an initial value. This would be no longer necessary technically, since we set all the variables in code (once this reliably works). But I guess there's no harm in keeping it (apart from the duplication).

[tarnung]

I rebased, removed the check for 'undefined' and added a default value for theme.

[munen]

@tarnung Really good work! I've added contribution to the changelog and added a section to the documentation, because this is too pretty to leave it off the docs(;