Dear Jscher2000:

Yes, it is the sidebar, and I actually keep it on the left side. i co nog know why I said it was on the right.

The color (but others nearby are OK): FAFA87

So this will involve using a userChrome.css file. I'm guessing you never created one before, but you'll find out as part of this process.

(A) You need to create a new chrome folder in your profile folder. This article has the steps for that (#1, #2, and optionally #3)

https://www.userchrome.org/how-create-userchrome-css.html

(B) Download the following file using right-click > Save Link As:

https://www.jeffersonscher.com/temp/userChrome-sidebar-background.css

Or if you open the file in a tab, right-click > Save Page As.

(C) Move the into that chrome folder

(D) Rename the file to just userChrome.css (that is, delete -sidebar-background from the file name)

The next time you quit Firefox and start it up again, it should discover that file and apply the rules.

May i recommend this add-on https://addons.mozilla.org/en-US/firefox/addon/new-tab-tools/. Once you install it, open a new tab click on the option gear in the upper right corner and select dark theme. Saved the hell out of my eyes.