This is mostly search and replace ala
GtkButton => button
GtkWindow => window
.button => button
or removing style properties that aren't used anymore like
-GtkButton-default-border: 0
If a side of the box is 0px wide, make the corners owned by the adjacent
sides. This avoids spilling over of unwanted colors from the 0-width
side into the corner.
New test for this case is included.