If you would like to add transparency to your menu elements, you can use rgba() colors. I do not recommend using opacity, as opacity will affect the entire menu (making it less legible when overlapping other items), while rgba() colors only affect the background color, not the foreground contents.
The WordPress Colorpicker does not currently support rgba() colors, so for the time being this needs to be done with custom CSS.
Here are some examples:
Black Menu Bar with .7 alpha value (30% transparency)
.ubermenu.ubermenu-horizontal{
background:rgba(0,0,0,.7);
}
Grey Submenu Background with .9 alpha value (10% transparency)
.ubermenu .ubermenu-item .ubermenu-submenu.ubermenu-submenu-drop{
background:rgba(240,240,240,.9);
}