[plt-scheme] extending MrEd

Does anyone have tips on where to begin if I wanted to do something like 
make my own Button class that has the background color as one of style 
options?  Or a toggle button that stays 'depressed' after you click it 
-- that sort of a thing.  I've looked through the source for mred but 
can't seem to find a starting point.  I'm confused as to where the mred 
classes meet the underlying code that actually draws a button and 
specifies things like the color.  I'm assuming the latter is not in 
Scheme but is in a library somewhere.  Any tips, pointers to documents, 
or thoughts on the feasibility of this?  Thanks in advance.

mike