Note that this is a global setting across all views, not just the current one.
The change should be instant, you will now see the darkened group headings.
Quick Tip: Darken the Outlook List Grouping Headers
One of the nicer features of Outlook 2007 is the way it neatly groups your messages by date or other criteria, but it's not always easy to see at a glance. There's a little-known option buried in the settings that will shade the group headings to make them more easy to distinguish.
