I went through and cleaned up old files after upgrading to 5.0.2, and I was in the Admin CP and saw an option for 'Installing Dark Mode.' I clicked on it and got a 500 error on the site almost immediately. I re-uploaded all the files, overwriting all existing files and I cannot get the forum to come up, install, upgrade - anything. It goes to "https://www.brianlumley.com/forum/admin/upgrade/?sessionCheck=1" with a 500 error. Unsure on how to fix.

Uploaded files to test directory and everything worked fine. Not sure what got corrupted, but after renaming the original folder and test folder, all is sorted and working now.

There is no "Installing dark mode" on the core platform. It sounds like you installed a 3rd party application that isnt compatible with the core product. Our product now contains a dark mode natively though, so you dont need to install anything to get dark mode