this is because right now they are not being centered.
To center the menu items you can add the rule below in Main Options > Fonts & Custom > Custom CSS rule box
.menu-primary .wvrx-menu-container li a {padding:0;text-align:center;line-height:2.8;}
padding:0 removes padding so centering is only done with true centering property, text-align:center centers horizontally, line-height does the vertical centering
You may also want to remove the separating line in the mobile menu with the rule below
.menu-arrows.is-mobile-menu a {border-none;}