I installed the latest BTT version from the BTT website but I still do have the UI of an older BTT version, not the polished one, one can see in the video of the Docs section.
My current BTT version is 3.390 (1613).
The "Use new BTT UI as default" option is selected.
I restarted BTT after installing it.
BTT is allowed to control my computer.
Deleting BTT preferences file (com.hegenberg.BetterTouchTool.plist) didn't help.
How can I get the BTT "new UI", not this one:
Thank you!
are you on an older version of macOS? The new UI is only available on 10.13 or newer
Thank you for your fast Sunday answer! 
I think my OS version is the culprit BUT I checked the OS requirements BEFORE downloading BTT on BTT website and all was OK as I have OSX 10.10.
At the moment (October 2019) BetterTouchTool works on macOS 10.10.x, 10.11.x, 10.12.x, 10.13.x, 10.14.x and up to the current macOS 10.15.4 Catalina. If you are still using macOS 10.6-10.9 you can download the appropriate BTT version from the downloads page.
There should be an OS requirements distinction between the versions with the "old UI" and the versions with the "new UI". The youtube video in the Docs section made me think all BTT versions have this UI now... And they don't.
I would have spared 1h30 of my time.
