Reinstate background colour for sidebars

This is required when the sidebar overlaps the map, for example when
it is not full height on the welcome message.

app/assets/stylesheets/common.scss

@@ -541,6 +541,7 @@ body.compact {
   #sidebar {
     float: left;
     width: $sidebarWidth;
+    background: #fff;
 
     #sidebar_loader {
       display: none;