I recall vaguely an article from years ago that explained we could develop dependent themes. E.g. one theme that inherits from another, and you would then only change certain files, while keeping the main theme as the basis. I am pretty certain it had to do with SS. Does anyone remember this? I googled back and forth and I'm not hitting anything.

Hereby I don't mean a "subtheme" for a module, but e.g. a theme that is derived from a main theme, but has a different look (e.g. different CSS's).

AFAIK inheritance isn't supported more than subthemes. You can also use the fact that if no theme contains the file mysite/templates is used so your 'base' could be mysite/templates/ then subthemes to themes/yourtheme

You can can use sub themes. They are not just for modules, they can be used to take a theme, and extend it for any page type. By using myTheme_page you can basically create a child theme that extends your master theme...

I've put some notes on this at http://www.silverstriperesources.com/articles/customising-a-theme-using-sub-themes/