The simpler media website CMS
I've read the documentation and done a search, but haven't discovered how best to specify custom CSS that tweaks a theme (Ardoise in my case) without editing the theme's own CSS file (screen.css for Ardoise).
For Gallery3, which I was using until recently, it was possible to point to a custom CSS file that was loaded so as to take precedence. This mean that when an updated version of a theme was installed, it wasn't necessary to rehack the theme's own CSS files. I note that Ardoise includes a custom.css file, but it's not clear what it does, because styling that I included in there seemed not to have any effect. So at present I've had to resort to modifying screen.css.