I would have written this as a comment, but I don't have enough reputation.
I would be very happy if you adopted Steve's cn1-css plugin into Codename One. I use it every day, and vastly prefer it to messing with the GUI theme/resource editor.
I use a lot of "custom" rounded rectangles with background color, and I'd much rather make a quick edit to a text file (.css) than "start over" in a paint application, divide image into pieces, etc. CSS is quicker and a lot less error prone (for me, at least).
Perhaps I am missing an "obvious" solution to this problem of "I want to try out a different bg color, or a different border radius," or maybe cn1-css is this obvious solution?
I am very grateful to Steve for writing this cn1-css plugin!