At this moment you can't change the opacity. All those colors for navigation and the like are hexadecimal numbers with 6 digits, like ffaa3e, for RGB. 00-FF for 0-255 in decimal.
Normally you need 8 digits to be able to set opacity, like ccffaa3e, where the first 2 digits (cc) can vary from 00 to ff (255).
For many colors in the map you can set this, but not for these colors.