Bug 1793543 - Never show a border on fieldset in in-content pages. r=aleca

Differential Revision: https://phabricator.services.mozilla.com/D158578

mail/themes/shared/mail/preferences/preferences.css

@@ -114,7 +114,7 @@ search-textbox {
 fieldset {
   margin: 0 0 32px;
   padding: 0;
-  border-style: none;
+  border-style: none !important;
 }
 
 fieldset > hbox,