Hey there Dustile,
This isn't a setting that is in the game, rather in the usersettings.txt.
Head to Documents\Elder Scrolls Online\Live and open the Usersettings.txt file in Notepad. Search for "Simple" and you should find a settings called "SET SIMPLE_SHADERS "X""
If X=1, simple shaders are enabled and you'll get that flat look. You can set X to 0 to return to the usual render style. In the end, the line would look like this:
SET SIMPLE_SHADERS "0"
We hope this helps!