[Theme: Travelify] Cannot Center Nav Menu and Sub Menu

For some reason I cannot center the horizontal nav menu. I have tried following the author’s CSS suggestions, and have managed to center top menu. However, when I follow second suggestion for sub menu it gets all messed up.

Here is the CSS tutorial from the author of the theme that I used.

https://colorlib.com/wp/forums/topic/how-to-center-menu-items-in-wordpress-travelify-theme

and here is my website

Thank you in advance for any help you are kind enough to provide!