Problem: The file tabs_on_bottom_menubar_on_top_patch.css is responsible for generating an empty space into which the menu bar is inserted. Unfortunately, on Macintosh systems, the menu bar is not inside the application window, but instead placed at the top of the entire display. This leaves an unsightly gap at the top of each window.

Solution: Add a media query to your import statement:

@import "hacks/tabs_on_bottom_menubar_on_top_patch.css" not (-moz-platform: macos);

This means the import will only run on platforms that are not macos (i.e: linux and windows).
Since tabs_on_bottom_menubar_on_top_patch is responsible for creating the extra space for the menubar, skipping import means that space is no longer created and therefore the appearance is improved.

Of course, media queries can also be used to have styling that is per-platform:

@media (-moz-platform: linux) {#someelement {background-image: "images/tux.png"}}
@media (-moz-platform: windows) {#someelement {background-image: "images/windows.png"}}

It actually, I think, looks pretty good on Mac, certainly more fitting than some of the other themes I've run.