Why isn't QScrollBar style applied in QT?

Question

Qt

There is a QTextEdit, you need a custom vertical scroll. I did it in the designer, I like everything, but after compilation the scrollbar remains default. Of course, nothing in the code changes the style. Thanks for the help.
UPDATE: To test, I took the stylesheet code from here doc.qt.io/qt-5/stylesheet-examples.html#customizin... , but it doesn't work.