The main problem here is that the difference between the grayed-out and the normal version is too subtle to see it if there is no direct comparison (e.g. a normal and a dimmed icon directly beside each other).
This esp. applies to the dark default theme
Most useful solution probably is to allow the theme author to make changes to the colors.
That way the normal and "grayed-out" state could be set to different colors making it absolutely clear what state the element is (e.g. selectable or not selectable)