I have been struggling these days with the updates to few of my themes due to the various places I have made changes to the default theme. I would like to suggest to have a search feature in debug mode which will allow me to search on specific keywords or phrases in all templates and return those in which the specified keyword/phrase exists.
When working with Google Chrome of Firebug you usually get the element you need and from there you see all the CSS rules which apply. From there I would like to simply paste the phrase/css rule and get the templates in which it is "found".
I guess if the general rule is the have ALL theme updates in the EXTRA.css or similar approach this feature will not be as important although I do think this approach as well as the one I used (changes in various templates) have their pluses and minuses.
Hope this makes sense.