Quid pro quo: not an experienced vB admin.
How would you normally debug this? Do you have, say, Firefox with Firebug installed?
Chrome also has a built-in debugger (which I think is there by default). IE's tool is a tad clunky, but I think it's available, at least on IE9 (try pushing F12 while on the page).
If you "inspect" an avatar, what do you see? The browser is rendering what it's told to render. How does the affected style render the HTML compared to the same spot in "the other theme(s)"?
edit: don't mean to sound "uppity" ... just need/wanted information to make an educated guess