Themes are made from CSS/LESS rules that you put in your user.less file. To use a theme open FoldingText based app and:
Choose the File > Open Application Folder menu item.
Inside that folder open your existing user.less text file, or create a new one if none exists.
Paste in the theme extensions CSS/LESS rules and save.
Create a new document to see the theme applied.
Is there a way to apply a theme to an existing file, or do you have to create a new document every time you define a new theme? For example, if I just want to change the font of an existing file?
If you want to apply a new them to an existing file you must restart FoldingText... then whatever new theme you set will be applied.