Fix transparent sidebar for very old browsers
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.
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