The theme uses the first (main) accent color as the navigation background color on hover. This is not independently configurable, but can be customize with CSS if desired:
body #access > div > ul > li:hover > a {
background-color: ...;
}
Thanks! I finally went with a different main accent color to resolve the problem. But it’s good to know how to change this. Since the menu closes up again if you try to inspect with it open, it’s difficult to find out.