You may prefer light mode if you prefer Squirrel Mail to Outlook Webmail. The light mode is usually quicker on slower computers and has a “Web Mail” style interface instead of a “Desktop” style interface.
To change to Outlook Web App Light, click the Options button in your mailbox.
In the Options page, click Settings then General. Under General, click Use the blind and low vision experience. Click Save, then click My Mail, then logout and login again to use Outlook Web App Light.