Its all done with javascript. It should be fairly easy to get the CSS. Jus' grab the head section, and search for the link, or inline stuff. Do that first, I'd say, then jus' display the result.
I'm in no way sure how you would change this actively,
unless you used the built-in support for scripts that Chrome has. The best guess I would say is: You reload the page, then change the CSS (The extension will run before the page is actually loaded).
I think.
For an example here, I have a small script running in FireFox which, on www.rockpapershotgun.com, takes the alt-text from images and displays it beneath them. Because alt-text isn't displayed as a tool-tip but Wordpress only provides opportunity to insert alt-text it makes life easier, because there are often funnies in the alt-text.
Whew! Basically, use Greasemonkey (scripts).