I cannot share the URL as I'm working on a local dev server you can't reach.
Generally speaking I'm not a fan of asking for admin access as the first step to solving problems, as it's often the case that the issue is something that can't be discovered without changing server settings, etc., that is not possible on a production server environment. This issue is a case in point:
I did some further testing and discovered the cause of the problem. I just reproduced the problem with a fresh install of your Quickstart package for J4, which ships with Jooma 4.2.4. With no other settings altered, I found both the "White Background" and "Dark Background" settings have no effect: they both result in the same subtle pattern seen with the "None" setting.
Then I tried changing my PHP version from 8.0 to 7.4 and the problem went away. So I'm guessing you tested it with an older version of PHP, perhaps?