HighContrast: Update treeview rubberband styling

Adapt to the changes in the previous commit.

gtk/theme/HighContrast/_common.scss

@@ -966,7 +966,7 @@ treeview.view {
   border-left-color: mix($fg_color, $base_color, 50%); // this is actually the tree lines color,
   border-top-color: $bg_color;                         // while this is the grid lines color, better then nothing
 
-  &.rubberband { @extend rubberband; } // to avoid borders being overridden by the previously set props
+  & rubberband { @extend rubberband; } // to avoid borders being overridden by the previously set props
 
   &:selected { border-radius: 0; } // rest border radius in lists
 

gtk/theme/HighContrast/gtk.css

@@ -109,7 +109,7 @@ textview text,
 .rubberband,
 rubberband,
 flowbox rubberband,
-treeview.view.rubberband {
+treeview.view rubberband {
   border: 1px solid #000;
   background-color: rgba(0, 0, 0, 0.2); }