Your code is correct, the reason you not able to see the style sheet applied is...

You are setting style for QSlider::groove:vertical, QSlider::handle:vertical, QSlider::add-page:vertical and QSlider::sub-page:vertical, all these are vertical components; and you are adding a new QSlider(Qt::Horizontal), which uses horizontal components

so either change you style to horizontal, or change QSlider to vertical