Bug 1487397 - Add hover styling on trees on Windows default theme. r=dao

Differential Revision: https://phabricator.services.mozilla.com/D5796

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

toolkit/themes/shared/tree.inc.css

@@ -29,6 +29,14 @@ treechildren::-moz-tree-row(multicol, odd) {
   background-color: -moz-oddtreerow;
 }
 
+%ifdef XP_WIN
+@media (-moz-windows-default-theme) {
+  treechildren::-moz-tree-row(hover) {
+    background-color: hsla(0,0%,50%,.15);
+  }
+}
+%endif
+
 treechildren::-moz-tree-row(selected) {
   background-color: -moz-cellhighlight;
 }