When I enter fullscreen mode, some themes don't fill the screen and the main part of the app doesn't fill the screen properly. I'm not sure if this is a bug or if it's to do with my monitor having an outdated aspect ratio, but I do remember older versions of the program not having this issue. I also have 'expand to window' selected in the options and I've tried reinstalling it. Does this seem like a problem with my monitor or will it need to be patched out?
I'm sorry for the trouble. In order for those themes to appear with the panel background image lined up with your monitor, Display Customizations > Expand Background to Window actually has to be turned off. The themes were designed before the option to expand the background existed, so when it's on it causes them to not look ideal visually. This is something I need to remedy in a future update of the program and the affected themes, especially since this option is on by default, and I appreciate you bringing it to my attention.