HighContrast: draw combo box separators

Same fix as the previous commit.

gtk/theme/HighContrast/_common.scss

@@ -755,7 +755,7 @@ GtkComboBox {
     color: $backdrop_insensitive_color;
   }
   .menuitem { text-shadow: none; }
-  .separator {
+  .separator.vertical {
     // always disable separators
     -GtkWidget-wide-separators: true;
     -GtkWidget-horizontal-separator: 0;

gtk/theme/HighContrast/gtk.css

@@ -911,9 +911,7 @@ GtkComboBox {
     color: #7f7f7f; }
   GtkComboBox .menuitem {
     text-shadow: none; }
-  GtkComboBox .separator, GtkComboBox GtkPlacesSidebar.sidebar .view .separator, GtkPlacesSidebar.sidebar .view GtkComboBox .separator,
-  GtkComboBox GtkPlacesSidebar.sidebar .view .separator:backdrop,
-  GtkPlacesSidebar.sidebar .view GtkComboBox .separator:backdrop {
+  GtkComboBox .separator.vertical, GtkComboBox GtkPlacesSidebar.sidebar .view .vertical.separator, GtkPlacesSidebar.sidebar .view GtkComboBox .vertical.separator {
     -GtkWidget-wide-separators: true;
     -GtkWidget-horizontal-separator: 0;
     -GtkWidget-vertical-separator: 0; }