Hey @Neil_Durant
Nice!
There’s nothing special about the built in themes and you should be able to create as many as you like - not sure why you can’t create a new one.
One quick tip - Cantabile will look for either a zip file renamed to .theme or a folder with .theme. Using a folder is easier while you’re editing the theme.
eg: I just extracted Dark.theme to a folder Dark2.theme:
and it shows up just fine:
Brad