I should mention; the splash screen works fine, and so does everything thereafter.
I tried downloading an SDDM theme, though upon learning I could manually change the background to the existing ones, I deleted it, then changed the Breeze theme’s background to a photo I have in Dropbox. I logged out, and and logging back in I just saw the american keyboard layout taking up most of my screen, the rest being black. I typed my password, which didn’t change the screen, until I submitted it, at which point the screen quickly flashed to a bizarre Windows 7-esque login (with my profile picture correctly shown) before getting to the splash screen, where everything started working fine. Is there a way to just reset this to the way it was? And beyond that, to apply a different background to the default Breeze theme without borking my login screen?
Edit: Bizarrely, the issue seemed to be that by deleting the custom theme, no theme was selected. That was an easy fix.
Now, though, it looks like the color of the password dots and “enter password” button… switched? The dots are so dark they’re barely visible, and the square button to the right of the password field is just a white square. Plus, logging out is now taking a good few seconds, whereas before it didn’t take nearly as long. I got the background working, at least.