Problem with dark mode on the MacOS Catalina 10.15.7

It sounds like something is confusing the OS.

Reboot your Mac and see if the problem is cleared. If rebooting your Mac doesn’t fix things, see if the problem still happens in Safe mode (hold the shift key down at startup). It can take much longer to safe boot (10 min) so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode forces the OS to clear certain caches and logs and do other miscellaneous housekeeping. It's non-destructive to user data so is safe to do. Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.

Does the problem persist while in Safe mode?

Exit safe mode by restarting your Mac normally and re-evaluate the issue again.

If the problem is still unresolved, please return here and post a reply and we’ll see what else we can come up with for help.

I know that CCleaner can be a life saver for Windows users, but the Mac is not Windows.

There is no need to use any "cleaner" of any sort. Some of them should be considered malware.

In any case, they tend to "clean" all too well, i.e. they delete stuff that should be left alone, and that can create all sorts of crashes, instability or bad behavior of your system.

My suspicion is that this may have been the case.

I recommend that you do a full backup, preferably two (I always have Time Machine up to date, and a clone of the system drive in a separate disk).

Then create a bootable installer, erase and install a clean system;

when it asks you migrate, choose "from another mac, drive or Time Machine"; attach your TM drive, but - CRUCIALLY - elect to import only the user accounts.

Having a backup is essential to anyone who cares about his or her data.

It is not something that should be left to chance. A hard disk is very inexpensive, and Time Machine is trivial to use.

It can be used to recover a few files, or everything. If you have a Time Machine backup done today you can easily restore to the situation you were in.

All drives will fail, eventually. Your mac may die, and all sorts of things can happen.

Without backups, losing data is a question of when, not if.

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

You should seriously get one - unless you don't mind losing your pictures, e-mails, and other data on your mac.

Not having a backup is a disaster waiting to happen.