Rewrite of CSS and HTML as well as minor fixes.

This is, for the most part, a rewrite of the CSS and HTML.
There was a lot of the CSS that was very badly formatted or not even used (I literally cut over 1000 lines from brass-min.css after formatting it) so I've tidied it up a bit.
I moved all relevant parts of both brass-min.css and the external default.css into a single stylesheet called style.css.

Besides fixing the CSS, I also did a few style changes/bug fixes to the website, such as fixing the border of the tables and the width of the pic-Rounded class (I assumed that it wasn't supposed to look like that).

I've updated the F-Droid icon to the latest version.

I've added an APK download to Firefox (I wanted to, and if it's okay, plan to add more APK downloads, but I didn't want to add that many features to this pull request, I just wanted to fix the CSS).