How change scrollbar knob "acceptance" area size?

[Gourmet]

I have customized vertical QScrollBar with reduced knob width. I did this using stylesheet. It works on touchscreen but it is too much thin to accept finger taps and move. I want keep it's visible size but increase area around where it accepts finger taps. On picture the white bar shows scrollbar knob and a doted border shows area when finger taps (or mouse clicks) must be accepted. Is it possible do this with just style sheet for this scrollbar? Or I have only inherit then rewrite this class?