Fix the placement of arrows in the zoom box

Now that [bug 649849](https://bugzilla.mozilla.org/show_bug.cgi?id=649849) has been fixed, adding support for `-moz-appearance: none`, the arrow is now too close to the text in the zoom box. This is currently only an issue in Nightly, but assuming the patch doesn't get backed out, this will soon affect all versions of Firefox.

The only simple solution I could find seems to be removing `*-appearance: none` rules from the CSS. I haven't been able to find any easier solutions that still looks the same with/without bug 649849.