Dark mode not working

If all else fails boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Read and follow the article linked above.

Note you have asked us to guess, among other things:

• The meaning of "not working"
• Your applicable settings in System Preferences > General
• Your applicable settings in System Preferences > Desktop & Screen Saver