Hello LordTermor, thanks for your help. All of the default themes are installed, as you can see here (currently using Breeze Twilight):
However, I’m getting strange artifacts in the kickoff menu, as well as the application menu, as you can see in the screenshots.
Additionally, this is what my desktop looks like with the Breeze theme active (note the color of the taskbar and menu remain black):
I haven’t done any customizations to my desktop theme whatsoever.