I reset my PC to troubleshoot problems, and in the meantime, I had an old beta of Aseprite installed in %appdata%. Then I got Steam and installed Aseprite (latest update). When I changed the theme, it said that a font named Aseprite LG was missing, but I was able to change the theme. When I close and relaunch, the theme resets every time.
I even cleared the config and removed the old beta version from %appdata%, and un- and reinstalling Aseprite via Steam to no avail.
How can I keep the theme? The theme is called Retro Dragon and the version of Aseprite I’m using is 1.2.40-x64.