Actually, @MrBillKnows, you can save Themes to a file.
File > Options > Manage > Save theme to file
Or
Window > Themes > Save theme to file
I change custom themes constantly and the file saved by this command works well for that. Change custom themes by using Load theme from file.
Side notes:
Save Options to file saves all your File > Options settings to a .prefs file. This is really good to do periodically as a backup, in case you have an unplanned uninstall/reinstall Scrivener situation.
Save theme to file saves all config aspects of a theme, including those that are not available from File > Options. This is what you want to use to save the settings for your customized themes and/or to share customized themes with others (as was done in this thread).
Save Theme Options to file ??? I assume it’s meant to save a subset of File > Options settings, but I really have no clue why or how it would be useful. It seems superfluous and redundant to the other two commands, and I wouldn’t be surprised if they do away with it.
Best,
Jim