Sub Menu Options:

Go to Appearance > Customize > Header >> Sub Menu section. There are options color in  Customize to configure for sub menu such: Background Color, Text Color, Text Hover Color.

submenu

After you finished customizing this section click the Save & Publish button.

How To Create Sub Menu:

The same configuration as section How To Setup A New Menu to add menu items into Main Menu. Then you drag the sub-menu item to the right until you see that the dashed outline box underneath is indented, then stop dragging:

dragging-first-sub-menu-item

Or simply use available options to move position:

submenu-2

Once you’ve moved your sub-menu items, remember to click Save Menu.