Had the same issue myself today, try the below fix, it corrected the problem for me. Thread: Changing windows settings - #19 by Rommax-1333
Fix Posted by wow dev Rommax:
Open your Config.wtf file in a text editor such as notepad. The default path is:
C:\Program Files (x86)\World of Warcraft\_retail_\WTF\Config.wtf
In that file, add a new line containing exactly:
SET ForceAllowAero "1"
Launching the client with this set will skip the code WoW runs to switch to the Basic theme while WoW is running.
Note when using this mode and running on DX12, the client may crash on launch if you don’t have a monitor plugged into the GPU the game tries to use. It will also likely crash if you unplug the monitor while WoW is running.
All of the above only applies to Windows 7.