Removed the day/night format option.

Instead of having a day/night pair for each theme, I have now included
a ‘dark’ theme and left all of the other themes as ‘light’ themes.
Fixed -webkit-transform pixellation issues.

First, there were pixellation issues on retina screens. When I applied
the `translateZ(0)` workaround, this started creating pixellation
issues on non-retina screens. So now the `translateZ(0)` workaround is
wrapped in a retina-only media query.