XojoThemes was originally made before Xojo supported themes natively. It allowed users to create their own themes and apply them to the code editor. It’s largely Sherlocked  now, but I’ve fixed a few things and rebuilt it as a Universal Mac app, in case anyone finds it useful. Xojo’s theme files use the same theme format as XojoThemes’ themes. That’s fun to say.
It was originally made in the pre-Dark Mode era, and while it functions, it hasn’t really been updated with Dark Mode in mind. I’ve also not rebuilt the Windows app, but do let me know if anyone is interested.
Thanks to @GarryPettet for the reminder.
Download the Mac app: https://www.libertyapp.com/xojothemes/XojoThemes.zip
Some theme files: GitHub - gavinlibertyapp/xojothemes: Themes for the Xojo IDE