Even if you have been using the new theme, switching to the new look from a notification at the bottom of the screen will slightly update you.
Gmail now has a fresh new look that matches Google's recent redesign. Conversation view is better, there are new search options and filters and the toolbar at the top has changed too.
If you are not comfortable with the spacious feel and think that there is too much whitespace, you can change that as well from the settings button on the top right.
I wish Google Reader gets this option too. I don't like the new look that came just a couple of days ago.
This is Google's official site explaining the new look and features.