Update _top-bar.scss

$topbar-link-dropdown-padding was overriding without the !default, making the variable as available in the settings file useless.

This is important to fix mostly because it breaks rem-based layouts (throws an error: "Incompatible uinits: 'px' and 'rem'" no matter what you do), but also because it is counterintuitive that if this is set in the settings file, it gets ignored.