I've recently changed the sidebar color in Nautilus to match the theme. Is there something I could do similar for Rhythmbox? Thanks for any help!
Yes, and it's not much different from changing the color of the Nautilus-Elementary sidebar.
Add the following to your
Then switch to a different theme and then back to your old one.
As you can see in the screenshot below, there is a border around the sidebar in Rhythmbox unlike in Nautilus-Elementary, but the color matching does improve how Rhythmbox looks, in my opinion.