I recently discovered this thing while I was checking about what actually guest account means..
I switched to GUEST ACCOUNT from settings. The theme of my phone changed to WHITE, i.e., similar to the one before the July 2018 patch update.
Then I removed the guest account to revert back to the ADMIN ACCOUNT. Then also, the theme remained the same, i.e., WHITE. It didn't changed to the original one, i.e., BLACK THEME.
I then restarted the device and everything was normal.
See this tweet where I had posted a video of the same: https://twitter.com/PraveenParihar1/status/1034832721284132864
I just want to ask whether any other Nokia 7 Plus user is facing any issue similar to this or not. I want to be sure whether the issue is common or only specific to my device. Nokia is not responding on Twitter, and the chat support in India is absolutely useless. The persons sitting on the other side have the same message copied to their clipboard which asks the user to FIRST CHECK USING THE CODE *#*#37 (some number), and if not satisfied, VISIT SERVICE CENTER.
Also, if someone know the way to REMOVE THE "ADD USER ICON" FROM NOTIFICATION PANEL THEN PLEASE TELL. There is NO OPTION AVAILABLE AS PER NOKIA. It leaves the phone at a serious risk as anybody can use it from lockscreen itself.