Sylvia - Highlighting a Widget

The  Sylvia demo site has one widget with a grey background colour ("Recipes + Travel in Your Inbox").

This can be done with a plugin called  Widget Options.

1. Install and activate the plugin.
2. Go to Appearance -> Widgets and edit one of your widgets.
3. You'll see new tabs at the bottom of the widget. Click the gear icon to switch tabs.
4. You'll see two input boxes: Widget CSS ID and Widget CSS Class. In the Widget CSS Class box, enter: highlight
5. Save.