$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.
$topbar-link-text-transform: none !default;
$topbar-link-padding: $topbar-height / 3 !default;
$topbar-back-link-size: rem-calc(18) !default;
-$topbar-link-dropdown-padding: 20px;
+$topbar-link-dropdown-padding: 20px !default;
$topbar-button-font-size: 0.75rem !default;
$topbar-button-top: 7px !default;