This adds a PlotWidget element to the QDarkStyle stylesheet so that axes labels in PyQtGraph plots aren't covered by unnecessary padding.

PyQtGraph QDarkStyle fix

from PyQt5 import QtCore, QtWidgets
import qdarkstyle

import pyqtgraph as pg

qss_plot_widget = r'''
PlotWidget {
    padding: 0px;
}
'''

app = QtWidgets.QApplication([])

class Plotter(QtWidgets.QWidget):
    def __init__(self, dark=False, fix_padding=False):
        super().__init__()

        self._layout = QtWidgets.QVBoxLayout()
        self._layout.setSpacing(0)
        self._layout.setContentsMargins(0, 0, 0, 0)
        self.setLayout(self._layout)

        self._plotter = pg.PlotWidget()
        self._plotter.plot(x=[1, 2, 3, 4], y=[1, 3, 2, 4])
        self._plotter.setLabels(title='Window 1', bottom='X Axis', left='Y Axis', right='Y2 Axis')
        self._plotter.setContentsMargins(0, 0, 0, 0)

        self._layout.addWidget(self._plotter)

        if dark:

            qss = qdarkstyle.load_stylesheet_pyqt5()

            if fix_padding:
                qss += qss_plot_widget
                self.setWindowTitle('QDarkStyle + PyQtGraph Fix')
            else:
                self.setWindowTitle('QDarkStyle')

            self.setStyleSheet(qss)
        else:
            self.setWindowTitle('No Style Sheet')



plot_light = Plotter(dark=False)
plot_dark = Plotter(dark=True)
plot_dark_fixed = Plotter(dark=True, fix_padding=True)

plot_light.show()
plot_dark.show()
plot_dark_fixed.show()

plot_light.move(QtCore.QPoint(10, 100))
plot_dark.move(QtCore.QPoint(400, 100))
plot_dark_fixed.move(QtCore.QPoint(800, 100))

app.exec_()