Change Mail sidebar background color?

hmibrooklyn wrote:

All of a sudden, the sidebar background color in Mail has changed from monochrome to a pale green. Anybody have an idea how I would get it back to default?

Have you tries shutting down the mac and restarting..?

Shut down or restart your Mac - Apple Support

A SafeBoot How to use safe mode on your Mac - Apple Support will sort many anomalies

The window is semi-transparent, so it will probably depend on whatever is behind it.

If you don't want that, you can select "reduce transparency" in System Settings->Accessibility.