Bug 1589129 - Fix about:preferences dialogboxes' close icon margin and focusring r=jaws

Differential Revision: https://phabricator.services.mozilla.com/D49451

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

browser/themes/shared/incontentprefs/preferences.inc.css

@@ -547,6 +547,8 @@ button > hbox > label {
   background: none !important;
   min-width: 0;
   min-height: auto;
+  padding-inline: 0;
+  margin-inline: 0 8px;
 }
 
 .dialogFrame {