Apply themes to ayop-logo.svg as well

The "themes" event handler now also applies the given CSS to ayop-logo.svg.
Add a dark theme to ayop-logo.svg

dark.css now also contains rules for ayop-logo.svg. Rules inside ayop-logo.svg have also been cleaned up.
Clean up themes event handler in ayop-main.js

Extracts the methods addCss and removeCss.
Fix a bug in ayop-preload.js if the SVG wasn't ready

fetchColors didn't check if the SVG document was ready before trying to fetch the colors; most of the time, this worked, but sometimes it failed. Now it should always work.