With the switch to CSS custom properties for the themes, old browsers
with no support are ending up with a transparent background on the
sidebar. This makes the site especially difficult to use on mobile.

I'm going to do something more extensive to allow browsers with no
support for custom properties to still get basic theming, but it's
dependent on a @supports query. For browsers that don't support that
query either, we need this line to give the sidebar a background.