Make theme colour (border below masthead) work

Ours is used to indicate the different
environments:
- local
- preview
- staging
- production

app/templates/admin_template.html

@@ -14,7 +14,7 @@
 {% block head %}
   <link rel="stylesheet" media="screen" href="{{ asset_url('stylesheets/main.css') }}" />
   <style>
-      #global-header-bar { background-color: {{header_colour}} }
+      .govuk-header__container { border-color: {{header_colour}} }
   </style>
   <meta name="google-site-verification" content="niWnSqImOWz6mVQTYqNb5tFK8HaKSB4b3ED4Z9gtUQ0" />
   {% block meta %}