WordPress 3.4: Features and Video
Today, Matt Mullenweg announced the availability of WordPress 3.4. WordPress includes significant improvements to the blogging application's theme theme customization, custom headers, Twitter embeds, and image captions.
More specifically with regards to theme customization, the new theme customizer allows you to play around with various looks and settings of your current theme and provides the ability to preview those settings without publishing the changes until you're ready to go public. For themes that support the theme customizer, you can also change colors, backgrounds, and image headers all from within the admin menu.