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.
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:
Or simply use available options to move position:
Once you’ve moved your sub-menu items, remember to click Save Menu.