".window-frame.solid.csd" - if you did this then it clearly wrong. It is ".window-frame.solid-csd", dash not point between solid and csd..solid-csd should not have rounded corners. It is intended to be used when there is no compositing manager and without compositing manager you can not have transparent rounded borders.
_GTK_FRAME_EXTENTS itself has noting to do with this. It is theme issue if you are testing without compositor otherwise most likely problem from ubuntu patch in GTK+.