Кастомизировать QScrollBar с помощью QSS

Я не знаю о каком QSlider вы говорите, но если вам надо кастомизировать QScrollBar с помощью QSS, тогда вот вам вариант:

from PyQt5     import QtWidgets, QtCore, QtGui


class InputBox(QtWidgets.QDialog):
    def __init__(self, parent= None):
        super(InputBox, self).__init__(parent)

        open_message = QtWidgets.QLabel("Enter Value:")
        self.txt = QtWidgets.QLineEdit()
        save = QtWidgets.QPushButton('Save', clicked=self.accept)
        cancel = QtWidgets.QPushButton('Cancel', clicked=self.reject)

        grid = QtWidgets.QGridLayout(self)
        grid.setSpacing(10)
        grid.addWidget(open_message, 0, 0)
        grid.addWidget(self.txt, 1, 0, 1, 2)
        grid.addWidget(save, 2, 0)
        grid.addWidget(cancel, 2, 1)
        self.setFixedSize(self.sizeHint())

    def save(self):
        value = self.txt.text()
        return value
        

class ListForm(QtWidgets.QWidget):
    def __init__(self, list_of_items, open_text):
        super(ListForm, self).__init__()

        open_message = QtWidgets.QLabel(open_text)
        self.lst = QtWidgets.QListWidget()
        self.lst.addItems([str(i) for i in list_of_items])
        add = QtWidgets.QPushButton('Add', clicked=self.add_button)

        grid = QtWidgets.QGridLayout(self)
        grid.setSpacing(10)
        grid.addWidget(open_message)
        grid.addWidget(self.lst)
        grid.addWidget(add)

    def add_button(self):
        input_box = InputBox()
        input_box.setWindowTitle("Window 2")
        if input_box.exec_() == QtWidgets.QDialog.Accepted:
            val = input_box.save()
            it = QtWidgets.QListWidgetItem(val)
            self.lst.addItem(it)
            self.lst.scrollToItem(it)


StyleSheet = """
QScrollBar:vertical {              
    border: none;
    background: white;
    width: 5px;               
    margin: 0px 0px 0px 0px;
    min-height: 0px;  
}
QScrollBar::handle:vertical {
    background: qlineargradient(x1:0, y1:0, x2:1, y2:0,
    stop: 0 rgb(32, 47, 130), stop: 0.5 rgb(32, 47, 130), stop:1 rgb(32, 47, 130));
    min-height: 0px;
}
QScrollBar::add-line:vertical {
    background: qlineargradient(x1:0, y1:0, x2:1, y2:0,
    stop: 0 rgb(32, 47, 130), stop: 0.5 rgb(32, 47, 130),  stop:1 rgb(32, 47, 130));
    height: 0px;
    subcontrol-position: bottom;
    subcontrol-origin: margin;
}
QScrollBar::sub-line:vertical {
    background: qlineargradient(x1:0, y1:0, x2:1, y2:0,
    stop: 0  rgb(32, 47, 130), stop: 0.5 rgb(32, 47, 130),  stop:1 rgb(32, 47, 130));
    height: 0 px;
    subcontrol-position: top;
    subcontrol-origin: margin;
}
"""
        

if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    app.setStyleSheet(StyleSheet)
    window = ListForm(list(range(20)), "List of Values")
    window.setWindowTitle('Window 1')
    window.show()
    sys.exit(app.exec_())