--- a/mail/themes/windows/mail/messageIcons.css
+++ b/mail/themes/windows/mail/messageIcons.css
@@ -2,19 +2,21 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 .treecol-image image {
   padding-inline-end: 4px;
   padding-bottom: 3px;
 }
 
-:-moz-system-metric(windows-classic) .treecol-image image {
-  padding-inline-end: 1px;
-  padding-bottom: 2px;
+@media (-moz-windows-classic) {
+  .treecol-image image {
+    padding-inline-end: 1px;
+    padding-bottom: 2px;
+  }
 }
 
 /* ..... column picker ..... */
 
 .tree-columnpicker-icon  {
   list-style-image: url(chrome://messenger/skin/icons/columnpicker.svg);
 }