[plt-scheme] Newbie question: Why "orange text, black background" after Run?

Thanks for your forbearance; I've searched the PLT Scheme mailing list  
and the Help Desk and haven't found anything useful.

Here's my problem. When I define some functions in the upper pane and  
press the Run button, the body of each function changes to orange text  
on a black background. Not only is this distracting per se, but it  
also makes it hard for me to see the green markers when I begin  
debugging the code.

I'm running PLT Scheme on MacOS X Leopard (10.5), and I've clicked  
Reset in the Preferences. The language used, noted at the bottom of  
the screen, is Advanced Student. I'm sure this must be something I'm  
not understanding; thanks for your help.

--Gregg