
This is discussed at some length on the Microsoft Answers site. I provide a couple of (unsatisfactory IMO) workarounds.
https://answers.microsoft.com/en-us/msoffice/forum/all/problem-with-quick-styles-gallery-display-change/3448286f-8f24-46d3-8fe4-5a497c67c104
I discuss a custom ribbon tab and adding things to your Quick Action Toolbar. It is very important that you leave feedback, though.
If you are using Office 365 on Windows 10, you may be able to roll this back, at least for a bit. This is discussed in the linked thread.