Bug 1577207 - Set margin of 0 for the overflow menu content in customize mode in non-osx;r=Gijs

There's a 4px margin from popup.css that applies here, which causes the content to get detached
from the arrow. When popup.css used to be loaded as a XBL sheet, this stylesheet wuoldn't have
applied to the customize mode overflow menu since it's not a real <panel> but just elements in
the document sharing the same structure.

Differential Revision: https://phabricator.services.mozilla.com/D45407

--HG--
extra : moz-landing-system : lando

browser/themes/shared/customizableui/customizeMode.inc.css

@@ -451,6 +451,7 @@ toolbarpaletteitem[place=toolbar] > toolbarspring {
   border-radius: var(--arrowpanel-border-radius);
 %else
   border: 1px solid var(--arrowpanel-border-color);
+  margin: 0;
   box-shadow: 0 0 4px hsla(0,0%,0%,.2);
 %endif
   max-width: 29em;