Adwaita: add .content-view:hover styling.

Just to make thumbs in various app hilight on hover.

gtk/theme/Adwaita/_common.scss

@@ -2161,9 +2161,9 @@ GtkSwitch {
 }
 
 //content view (grid/list)
-
 .content-view {
   background-color: darken($bg_color,7%);
+  &:hover { -gtk-image-effect: highlight; }
   &:backdrop { background-color: darken($bg_color,7%); }
 }
 

gtk/theme/Adwaita/gtk-contained-dark.css

@@ -2823,6 +2823,8 @@ GtkSwitch {
 
 .content-view {
   background-color: #282c2c; }
+  .content-view:hover {
+    -gtk-image-effect: highlight; }
   .content-view:backdrop {
     background-color: #282c2c; }
 

gtk/theme/Adwaita/gtk-contained.css

@@ -2985,6 +2985,8 @@ GtkSwitch {
 
 .content-view {
   background-color: #dbdbdb; }
+  .content-view:hover {
+    -gtk-image-effect: highlight; }
   .content-view:backdrop {
     background-color: #dbdbdb; }