How to make reading panel and mail content dark in Dark theme?

I’m using Mailbird and plan to move to emclient since MB doesn’t support IME input in Windows 10. However, I found that the Dark theme doesn’t make the email content dark as Maiilbird did. 
Does any one know how to solve this? I really need the dark theme.
https://drive.google.com/file/d/1_YbBcARYeH6d8ifnALtbJUbHtVSbOTBn/view?usp=drivesdk

Was this not provided in a recent release 7.2.33939?