Since latest "nb169 (July 11, 2018)" release by AI2, I got issue when setting Background Color to "None" for buttons. Buttons do no longer turn trasnparent by setting Background Color to None. Se exeamples in attached.
I was abble to handle it by difining a trasnparent image to the button, but I do not like this kind of work arround, so I hope MIT will fix that soon.