Caveat: I don't have a NUC, and I've forgotten more about Unix/Linux than I remember!
Maybe the "Browse Themes folder" functionality doesn't work, but the rest of it does. Are you able to click on the current (Default) theme and select "No Theme"?
If so, you can probably manually put a theme in place and use it.
On Windows the themes folder is in "C:\Program Files\J River\Media Center 23\Skins\Theater View\Themes". If you can get to the equivalent folder on the NUC using the File Manager it must include, you may be able to add a new subdirectory for the theme you want to set up. You just need to have a subdirectory full of the images you want to use for the theme, and that subdirectory will appear as one of the themes you can select in the MC Options.
For example, I have two custom themes in "\Themes\Puppies" and "\Themes\Dakar", alongside the standard "\Themes\Default" folder/theme.
Give that a try.