gtk theme question

i've done my searching but no topic about gtk themeing seems to point to any proper documentation explaining what every property does (from hours of googling, I can say that there doesn't seem to be any).

I want to get rid of the bevel/shadow effects, bits of which I traced in green below.

I thought that

GtkWidget::shadow_type = GTK_SHADOW_NONE

was supposed to do the trick but that wasn't the case. Apparently there's a gtk engine named flat, which sounds like it might be doing exactly this, but I couldn't find any documentation on it, let alone a bad one. From what I understand if I use pixmap the images will replace all that, but I'm not even decided on the color scheme yet so I don't want to get into the image creation business at this stage.

Any ideas?