Bug 30952: Fix background of header search "extra-content" toggle

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

koha-tmpl/intranet-tmpl/prog/css/src/_header.scss

@@ -172,6 +172,7 @@ a.navbar-toggle {
 
     .form-extra-content-toggle {
         opacity: .5;
+        background-color: transparent;
 
         &:hover {
             opacity: 1;