Remove background color from logo on mobile

app/assets/stylesheets/layout/_all.sass

@@ -30,6 +30,7 @@
 @import layout/base_mobile
 @import layout/grid
 @import layout/top_menu
+@import layout/top_menu_mobile
 @import layout/breadcrumb
 @import layout/main_menu
 @import layout/footer

app/assets/stylesheets/layout/_top_menu_mobile.sass

@@ -0,0 +1,31 @@
+//-- copyright
+// OpenProject is a project management system.
+// Copyright (C) 2012-2015 the OpenProject Foundation (OPF)
+//
+// This program is free software; you can redistribute it and/or
+// modify it under the terms of the GNU General Public License version 3.
+//
+// OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+// Copyright (C) 2006-2013 Jean-Philippe Lang
+// Copyright (C) 2010-2013 the ChiliProject Team
+//
+// This program is free software; you can redistribute it and/or
+// modify it under the terms of the GNU General Public License
+// as published by the Free Software Foundation; either version 2
+// of the License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+//
+// See doc/COPYRIGHT.rdoc for more details.
+//++
+
+@include breakpoint(680px down)
+  #logo
+    background-color: transparent