It is not possible to change color of the dropdown with just uss class. So Luna UI provides a uss variable you can change.
/* Dropdown.uss file in Samples/SetupExample *//* USS Variables you can override */:root { --color-dropdown-bg:var(--color-slate-600); --color-dropdown-hover:var(--color-slate-400); --color-dropdown-active:var(--color-red-400);}