Implement 134

Implements #134.

DO NOT MERGE YET. The toggle checkbox works, but I'm not happy with the CSS itself - it moves some stuff around that it IMHO shouldn't move around. Since I have literally *no* experience with CSS, I could use some feedback here.

Apart from that, I'm quite happy with how the themes are implemented now - if you follow the rules outlined in index.html, no javascript changes are necessary to add more themes.

There are also some minor changes (add trailing newlines, dos2unix).