Adding an ID to toolbar buttons

Sorry to re-post from the other forum, but I obviously posted in the wrong place...

I'm trying to re-skin the toolbar for a project I'm working on and would like to be able to apply custom CSS to individual toolbar buttons (e.g. to use larger images for specific buttons).

Ideally, I'd like to be able to re-work the whole toolbar HTML but it's buried in so many different places I'll settle just for being able to apply ID attributes to each button where the ID is set to the button name used in defining the toolbars, e.g. id="SpellCheck"

I'm having trouble even finding where to do this though. I just need to be able to add the id attribute to either the icon img tag or the surrounding td.

Any help is much appreciated!