Is there a way to "rip" Qt stylesheet from Python app? 
.styleSheet() returns an empty string.
You're assuming two things:

The app has implemented its styling using stylesheets, and
The app uses an application-wide stylesheet.

Either of these assumptions is wrong in your case. You might need to extract stylesheets from individual widgets. Or the application might be using a custom QStyle and not stylesheets.
Get Qt stylesheet from Python app