Outlook Web App only uses light version in IE

Had a patron that could not "uncheck" Use the light version of Outlook Web App
on their login screen.

They had Internet Explorer 11 on Windows 7.   Solution is to use Compatability View:

To do this, press F10 to display the menu bar, go to the Tools menu in Internet Explorer 11, and then click Compatibility View settings. Then, add the OWA site to the list of sites to be viewed in compatibility view.

From Microsoft Knowledge Base