Use nesting for thumbnail styles

less/thumbnails.less

@@ -14,18 +14,17 @@
     margin-left: auto;
     margin-right: auto;
   }
-}
-
 
-// Add a hover state for linked versions only
-a.thumbnail:hover,
-a.thumbnail:focus,
-a.thumbnail.active {
+  // Add a hover state for linked versions only
+  a&:hover,
+  a&:focus,
+  a&.active {
     border-color: @link-color;
-}
+  }
 
-// Image captions
-.thumbnail .caption {
+  // Image captions
+  .caption {
     padding: @thumbnail-caption-padding;
     color: @thumbnail-caption-color;
+  }
 }