Issue #1793 - Add a border-bottom to treecols header

toolkit/themes/windows/global/tree.css

@@ -198,6 +198,14 @@ treechildren::-moz-tree-cell-text(progressmeter) {
 
 /* ::::: tree columns ::::: */
 
+@media (-moz-os-version: windows-win10) and (-moz-windows-theme: aero) {
+  treecols {
+    /* Windows 10's "aero" msstyle does not provide a complete style for
+       the treecols header so we need to provide a bottom border */
+    border-bottom: 1px solid ThreeDLightShadow;
+  }
+}
+
 treecol,
 treecolpicker {
   -moz-appearance: treeheadercell;