Combo widget does not support background color property so there is no
alpha channel. However this feature is on my list and I added your vote
to it. Let me know what other widgets you think should have background
Eric Martel12 Sep 2012
Well, thanks! :) In my case at least I don't need this feature for other widgets right now, but I guess it would be nice in general to have it for any widget that does have some kind of background (like Button, Accordion, etc.).