I don't agree that Common.css is only for article styling, but ignoring that...
How can you use Less to enforce scoping?
What prevents someone from putting this in Common.css?
} body { color: red;
or this?
a.my_custom_navlink { position: absolute; top: -50px; left: -50px; }
The first example should set body text to red by closing the scope you wrapped it with. The second example should push a link above and to the left of the article content.
If you want to limit CSS, I think you need to parse it, whitelist specific properties and values, and prepend a content div selector to all selectors.