Submenu issue on mobile…

Is there a way to add a dash, arrow, other color, or indent to submenus on mobile devices?

As it is, there is no way on a mobile device to distinguish between a submenu and main menu item once the submenu is open. All menu items (both main and sub) align together. This makes for an odd appearance.

I have even tried adding a dash and/or an arrow to the actual display title of each submenu within WordPress, however these characters are deleted when the menu is displayed (on both mobile and desktop).

Is there some simple add-on CSS I could use to give submenus a different look and distinguish them from main menu items on mobile devices?