Additionally, there are also two older themes. Note: They might not get updated frequently and are kept for legacy reasons:
GitHub Light (legacy)
GitHub Dark (legacy)
Override this theme
To override this (or any other) theme in your personal config file, please follow the guide in the color theme documentation. This is handy for small tweaks to the theme without having to fork and maintain your own theme.
Press F5 to open a new window with your extension loaded
Open Code > Preferences > Color Theme [⌘k ⌘t] and pick the "GitHub ..." theme you want to test. Note: It seems this has to be done 2x because the first time it switches back to the default light theme. This might be a bug?
UI: For all changes to the "outer UI", like (status bar, file navigation etc.), take a look at the Theme Color reference.
Syntax: For changes to the "code highlighting", examine the syntax scopes by invoking the Developer: Inspect Editor Tokens and Scopes command from the Command Palette (Ctrl+Shift+P or Cmd+Shift+P on Mac) in the Extension Development Host window.
Run yarn build to update the theme. You can also run yarn start instead to automatically rebuild the theme while making changes and no reloading should be necessary.
Once you're happy, commit your changes and open a PR.