It's very easy to change the backgound color of XWidget widgets.
I would like to take Yosemite Weather for example to make the method clear.
First of all, you should use XWdiget's SolidColor attribute to build the background rather than images.
To get a rounded rectangle background, you can set the roundness to 5px.
Add a new menuitem and type "!SelectBgColor=BG" in the OnClick input box. BG is the name of the image component BG.
That's all. As I mentioned ahead, it's very easy.
You can download the example here: