I wondered if anyone else has seen this bug. I just reported it.
It might be a good idea to turn off Sync preferences before you try this. Here's what you do:
Open Preferences>Appearance and set the UI brightness all the way to lighter. Click OK and the UI is now set to the lightest setting. Now open Preferences>Appearance again and move the UI Brightness all the way to darker or press the Reset button. The UI changes back to darker. Click OK and the UI returns to the lightest setting. Open up Preferences>Appearance again and the slider is all the way to the left and pinned on Darker but the UI is still light. Touch the slider and the UI changes as it should but the changes will not stick. The only way I have found to get the UI back to my preferred darker setting is to delete the preferences file. This is easiest do to by holding down all of the keys on the right side of the keyboard (Shift + Alt/Option + Ctrl/Cmnd) while you open AE and saying yes to the popup. This is a bit of a pain because I have created a bunch of custom settings for Output Module and Render Settings that all have to be redone. Unfortunately I had sync turned on so it fouled up all of the machines I work on before I realized what had happened.
Please let me know if you have seen this bug and report it to Adobe if you have.
Thanks for letting us know about this, Rick.