If you have never chosen to use Outlook Web App Light, you may be using Light mode because you are using an unsupported browser for Premium Mode. Unsupported browsers include IE6, Safari on Windows and Opera. Premium Mode is supported on IE 7 or higher, recent versions of Firefox and Mac OS X Safari 4 or higher.
If you have chosen to use Light mode and wish to switch back to Premium, click Options in your mailbox.
In the options pages, choose Accessibility. Unclick “Use the blind and low vision experience”, press Save then, click Mail then logout and login again.