How to use the harvesters._private.frontend.pyqt5.helper.get_system_font function in harvesters

content += '\n\n'
        content += 'PyQt5 (GPL)\n'
        content += 'Copyright (c) 2018 Riverbank Computing Limited\n'
        content += 'https://www.riverbankcomputing.com/'
        content += '\n\n'
        content += 'Icons8 (Creative Commons Attribution-NoDerivs 3.0 Unported)\n'
        content += 'Copyright (c) Icons8 LLC\n'
        content += 'https://icons8.comn'
        content += '\n\n'
        content += 'Versioneer (Public Domain, CC0-1.0)\n'
        content += 'Copyright (c) 2018 Brian Warner\n'
        content += 'https://github.com/warner/python-versioneer'

        self._text = QPlainTextEdit(content)
        self._text.setReadOnly(True)
        self._text.setFont(get_system_font())
        self._text.setLineWrapMode(True)
        self._text.setFixedWidth(480)

        layout.addWidget(self._text)
        self.setLayout(layout)

elif interface_type == EInterfaceType.intfIEnumeration:
            w = QComboBox(parent)
            for item in feature.entries:
                w.addItem(item.symbolic)
            w.setCurrentText(feature.value)
        elif interface_type == EInterfaceType.intfIString:
            w = QLineEdit(parent)
            w.setText(feature.value)
        elif interface_type == EInterfaceType.intfIFloat:
            w = QLineEdit(parent)
            w.setText(str(feature.value))
        else:
            return None

        #
        w.setFont(get_system_font())

        return w

def __init__(self, parent=None):
        super().__init__(parent)
        self.setFont(get_system_font())

def _setup_toolbars(self):
        #
        group_filter = self.addToolBar('Node Visibility')
        group_manipulation = self.addToolBar('Node Tree Manipulation')

        #
        label_visibility = QLabel()
        label_visibility.setText('Visibility')
        label_visibility.setFont(get_system_font())

        #
        self._combo_box_visibility = QComboBox()
        self._combo_box_visibility.setSizeAdjustPolicy(
            QComboBox.AdjustToContents
        )

        items = ('Beginner', 'Expert', 'Guru', 'All')
        for item in items:
            self._combo_box_visibility.addItem(item)

        shortcut_key = 'Ctrl+v'
        shortcut = QShortcut(QKeySequence(shortcut_key), self)

        def show_popup():
            self._combo_box_visibility.showPopup()

def _initialize_widgets(self):
        #
        self.setWindowIcon(Icon('genicam_logo_i.png'))

        #
        self.setWindowTitle('GenICam.Harvester')
        self.setFont(get_system_font())

        #
        self.statusBar().showMessage('')
        self.statusBar().setFont(get_system_font())

        #
        self._initialize_gui_toolbar(self._observer_widgets)

        #
        self.setCentralWidget(self.canvas.native)

        #
        self.resize(800, 600)

        # Place it in the center.
        rectangle = self.frameGeometry()

def _initialize_widgets(self):
        #
        self.setWindowIcon(Icon('genicam_logo_i.png'))

        #
        self.setWindowTitle('GenICam.Harvester')
        self.setFont(get_system_font())

        #
        self.statusBar().showMessage('')
        self.statusBar().setFont(get_system_font())

        #
        self._initialize_gui_toolbar(self._observer_widgets)

        #
        self.setCentralWidget(self.canvas.native)

        #
        self.resize(800, 600)

        # Place it in the center.
        rectangle = self.frameGeometry()
        coordinate = QDesktopWidget().availableGeometry().center()
        rectangle.moveCenter(coordinate)
        self.move(rectangle.topLeft())

items = ('Beginner', 'Expert', 'Guru', 'All')
        for item in items:
            self._combo_box_visibility.addItem(item)

        shortcut_key = 'Ctrl+v'
        shortcut = QShortcut(QKeySequence(shortcut_key), self)

        def show_popup():
            self._combo_box_visibility.showPopup()

        shortcut.activated.connect(show_popup)

        self._combo_box_visibility.setToolTip(
            compose_tooltip('Filter the nodes to show', shortcut_key)
        )
        self._combo_box_visibility.setFont(get_system_font())
        self._combo_box_visibility.currentIndexChanged.connect(
            self._invalidate_feature_tree_by_visibility
        )

        #
        button_expand_all = ActionExpandAll(
            icon='expand.png', title='Expand All', parent=self,
            action=self.action_on_expand_all
        )
        shortcut_key = 'Ctrl+e'
        button_expand_all.setToolTip(
            compose_tooltip('Expand the node tree', shortcut_key)
        )
        button_expand_all.setShortcut(shortcut_key)
        button_expand_all.toggle()