@justinmulligan, your main problem is the same as problem #1 in my post #2 in this topic: [18.1.11] Added "Form Defaults" section and "Custom Theme" checkbox - #2. It was fixed, as you can see from @lubos’s post a few further down in that thread.
But you seem to have a secondary problem that may be causing the first to manifest again. Can you go to Settings => Themes and post a screen shot of your list of themes? You only need to include the portion of the list including active themes (in black text).