At Devcon their was a conversation in one of the sessions about best practices for migrating your existing layouts from FM11 to Fm12 in order to not create CSS bloat. This information was not on a slide and so I can no longer find it (or my notes)
It went something like this...
Step 1 - Convert your solution
Step 2 - Go to a layout, select all objects and drag them off the screen
Step 3 - With all the items still selected, click the 'remove styles' button from the inspector
Step 4 - Now Undo the last action with a Ctrl Z to re-apply the look & feel using FM12 CSS
Step 5 - Now drag the objects 1 at a time back onto the layout and see how they look, re-apply styles only as neccessary.
I think those are all the steps but I am not sure I have them in the correct order - anyone?