Apply dark theme to ayop-logo.svg as well

I didn't like how the logo looked in the dark theme (the full-blown white clock face looked a bit aggressive), so themes are now applied to that as well. dark.css already contains a few rules for that, so that the logo looks now pleasant in both bright and dark layouts.

I also cleaned up the code a bit, and fixed a stray bug in ayop-preload.js.

I also thought about applying themes to preloadindicator_legend.svg, and thus allowing themes to change the colors of the preload indicator, but now I'm not sure if it's a good idea: We would have to completely redraw the preload indicator on each theme change...  what do you think? Would this be a nice feature or unnessecary? (If you want it, we can put it into this pull request as well.)