Hi,
I am sorry for the late response. have you figure out how to change the style?
the submenu styling should be on the style panel “submenu panel style” and change on mobile mode. click on the device icon in the top right corner of the editor. select mobile and go to the panel once again. to know which options can be styled in different device modes, there should also be a device icon beside that option.
let me know if you have more questions.