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).
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).