He's talking about dynamically adding classes to stylesheets, not
adding existing classes to elements.
My long-view recommendation is to point to a per-session stylesheet in
your markup (give it a very long cache lifetime, so it's not an
appreciable bandwidth hit) that starts out empty on the server, rather
than instantiating the STYLE element yourself.
For your initial rollout, add your rules to the stylesheet at runtime,
understanding you'll be discarding them on every new page view. This
gives you a good migration path for the next step: persisting session
styles onto the server, so you don't have to re-add rules unless you
get an onresize or a new session. (Or you can use client-side
persistence methods if you want.)
A great guide to the technicals is here:
http://www.hunlock.com/blogs/Totally_Pwn_CSS_with_Javascript
-- Sandy
