HighContrast: Update flowbox rubberband styling

Adapt to the changes in the previous commit.

gtk/theme/HighContrast/_common.scss

@@ -126,6 +126,10 @@ rubberband {
   background-color: transparentize($selected_bg_color,0.8);
 }
 
+flowbox {
+  & rubberband { @extend rubberband; }
+}
+
 label {
   &:selected,
   &:selected:focus,

gtk/theme/HighContrast/gtk.css

@@ -108,6 +108,7 @@ textview text,
 
 .rubberband,
 rubberband,
+flowbox rubberband,
 treeview.view.rubberband {
   border: 1px solid #000;
   background-color: rgba(0, 0, 0, 0.2); }