Changing sidebar background colour in Pilcrow?

Hi, I’m wondering if anyone can help. I’m using Pilcrow with two sidebar columns on the right of the main body and I’d like to try putting a background colour behind the sidebars to differentiate them from the blog content.

I know I’m looking for #sidebar in the stylesheet (and have found ten instances of it), but I can’t find a hex code listed anywhere.

Can anyone tell me which bit I need to fiddle with in order to try different colours? This is my website – https://www.theskullillusion.com (please be aware that it contains vintage postmortem photography and might not be to everyone’s taste).

I’d really appreciate any advice. Vi x