Would it be possible to add an option for adding custom syntax themes without the need to integrate it with the Zola core? something like what already exists for the syntax files i.e. extra_syntaxes. The one for syntax themes could be called extra_syntax_themes.
Long term it could be better to remove that entirely from the core and let users specify the syntax themes by putting a proper tmTheme file in their websites directly. And maybe Zola could generate the default one for starters.