Use Background fill color in Panel settings:

The CSS would be:

fieldset { background-color: rgb(200, 200, 200); }‍

Or if you prefer to define a class for your panel (e.g. 'foo'):

fieldset.foo { background-color: rgb(200, 200, 200); }‍

Now how simple was that? I was certainly overthinking it!!!

Very useful to know...thank you!

Just in case others might be searching for this in future, the answer marked as "correct" does not show you how to change a panel's background colour using CSS (as per the subject of the thread). Rather, it shows how it can be done via the form designer toolbar.

So, if you're wanting to know how to change the background colour of a panel using CSS, then refer to my answer above.