UI dropdowns: Selected option has purple highlight (#45254)

For the following bug:
- https://github.com/fleetdm/fleet/issues/45248

frontend/components/forms/fields/Dropdown/_styles.scss

@@ -240,7 +240,7 @@
     border-radius: $border-radius;
 
     &.is-selected {
-      background-color: $ui-vibrant-blue-10;
+      background-color: $ui-fleet-black-5;
       font-weight: bold;
     }
 

frontend/components/forms/fields/DropdownWrapper/DropdownWrapper.tsx

@@ -131,7 +131,7 @@ const getOptionBackgroundColor = (
   state: OptionProps<CustomOptionType, false>
 ) => {
   if (state.isFocused) return COLORS["ui-fleet-black-5"];
-  if (state.isSelected) return COLORS["ui-vibrant-blue-10"];
+  if (state.isSelected) return COLORS["ui-fleet-black-10"];
   return "transparent";
 };