I want to create my own theme using the JSON format (i.e. .sublime-theme, not .tmTheme) not the XML format.
I don’t want to have to start from scratch. What I’d like to be able to do is to have the default theme as JSON and then just edit that… however I can’t seem to find this anywhere.
Any clues?
Assuming you actually mean sublime-theme files, you can use View Package File from the command palette and enter Default theme to find and open Theme - Default/Default.sublime-theme to open a read only copy of the default theme. From there you could create your own theme using that as a base. You can also use that command and enter for Adaptive theme to get at that theme as well.
That said, sublime-theme files have always been JSON files, and while tmTheme files are indeed XML plist files, they were superseded by sublime-color-scheme files (which are also JSON). Despite the name, tmTheme files are color scheme files, not application theme files.
If your intention is to theme the application as a whole (change tab size and shape, change the fonts and colors used in panels, etc) then the above will get you what you want. If on the other hand you want to customize syntax highlighting colors, you want sublime-color-scheme files instead. In which case you can use the same command as above and enter sublime-color-scheme as the text to see all of the color schemes that are currently installed.
You can also search for and open tmTheme files the same way; while you have one open you can choose Convert Color Scheme from the command palette to convert from the old style color scheme to the new one.