Hi Frank,
All colors in the Customizer are removable, which achieves full transparency. But you’re right, there’s no partial transparency feature.
In order for colors to be semi transparent, an RGBA value must be set instead of a hex. In order for me to achieve this, I would have to find a way to convert hex values into RGBA – difficult, but may be possible at some point.
Ah yea, I wanted to cut down on the amount of options, and figured those were less needed for nav items vs everywhere else. I may have to add them π