When I change the background opacity setting to some value less than 100 only the top and the right areas of the menu are affected, the left one (the one that lists the apps in the category selected at its right) has a 100% opacity.
That's actually on purpose. I found that any transparency at all made it so you couldn't read the application names. If I could force a blur behind the window, that would be different, but that's not something I can do (that would be handled by the window manager).