HighContrast: Update the frame styling

Adapt to the changes in the previous commit.

gtk/theme/HighContrast/_common.scss

@@ -1904,6 +1904,7 @@ GtkLevelBar.vertical {
 /**********
  * Frames *
  **********/
+frame,
 .frame {
   border: 1px solid $borders_color;
   &.flat { border-style: none; }

gtk/theme/HighContrast/gtk.css

@@ -2101,13 +2101,17 @@ calendar:hover:selected, label:selected, label:selected:focus, label:selected:ho
 /**********
  * Frames *
  **********/
+frame,
 .frame {
   border: 1px solid gray;
   padding: 0; }
+  frame.flat,
   .frame.flat {
     border-style: none; }
+  frame:backdrop,
   .frame:backdrop {
     border-color: #8d8d8d; }
+  frame.action-bar,
   .frame.action-bar {
     padding: 6px;
     border-width: 1px 0 0; }