Its taken me some time create a base theme..but ShelfLife code feels like it. So yeah, learning one could say, but I feel the latests themes have been more complete in terms of design, more than earlier things.
Pushed the boundaries..I don't know about that though. I favored learning the PHP bit just as much as the HTML/CSS, that made it easy for me to change the templates, especially for SMF 1.1. It looked ok with all those tables no matter what I threw at it.
But looking through my old themes now, most of them seems a bit clunky and odd, tbh.
I guess its a matter of perspective.