HighContrast: Update switch styling

Use the new element names instead of the type name and style
classes, just like we did for Adwaita in the previous commit.

gtk/theme/HighContrast/_common.scss

@@ -1559,30 +1559,32 @@ GtkTreeView.view.progressbar { //Progressbar in treeview cells
 GtkSwitch {
     -GtkSwitch-slider-width: 47px;
     -GtkSwitch-slider-height: 28px;
+}
+
+switch {
     font: bold condensed 9;
     outline-offset: -4px;
-    &.trough {
-      // similar to the .scale
-      border-width: 1px;
-      border-style: solid;
-      border-radius: 3px;
-      border-color: $borders_color;
-      background-color: mix($bg_color,$borders_color,60%);
-      box-shadow: inset 0 1px transparentize(black, 0.9); /*outset not working*/
-      text-shadow: 0 1px transparentize(black, 0.9);
-      &:active {
+    // similar to the .scale
+    border-width: 1px;
+    border-style: solid;
+    border-radius: 3px;
+    border-color: $borders_color;
+    background-color: mix($bg_color,$borders_color,60%);
+    box-shadow: inset 0 1px transparentize(black, 0.9); /*outset not working*/
+    text-shadow: 0 1px transparentize(black, 0.9);
+    &:active {
         color: white;
         border-color: $selected_borders_color;
         background-color: $selected_bg_color;
         text-shadow: 0 0 2px white;
-      }
-      &:insensitive {
+    }
+    &:insensitive {
         color: $insensitive_fg_color;
         border-color: $borders_color;
         background-color: $insensitive_bg_color;
         text-shadow: none;
-      }
-      &:backdrop {
+    }
+    &:backdrop {
         color: $backdrop_fg_color;
         border-color: $backdrop_borders_color;
         background-color: $backdrop_bg_color;
@@ -1599,9 +1601,8 @@ GtkSwitch {
           color: $insensitive_fg_color;
           border-color: $insensitive_fg_color;
         }
-      }
     }
-    &.slider {
+    & slider {
       border-radius: 3px;
       @include button(normal, $edge: none);
       box-shadow: inset 0 1px white,