Implement 134: A toggle button for a dark theme

Implements and closes #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.~~
I "fixed" the CSS by removing any rules that touched layout or fonts. Feedback still appreciated.

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) - because of the latter, I recommend viewing this pull request in [Ignore Whitespace Mode](https://github.com/deplicator/xkcdTime_atyourownpace/pull/138?w=1) (`?w=1`).