myWindow.py

from PyQt5.QtCore import Qt
from PyQt5.QtGui import QPalette, QColor
from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QHBoxLayout

def newWindow():
    app = QApplication([])
    mywindow = QWidget()

    # Dark Theme
    app.setStyle("Fusion")
    dark_palette = QPalette()
    dark_palette.setColor(QPalette.Window, QColor(53, 53, 53))
    dark_palette.setColor(QPalette.WindowText, Qt.white)
    dark_palette.setColor(QPalette.Base, QColor(25, 25, 25))
    dark_palette.setColor(QPalette.AlternateBase, QColor(53, 53, 53))
    dark_palette.setColor(QPalette.ToolTipBase, Qt.white)
    dark_palette.setColor(QPalette.ToolTipText, Qt.white)
    dark_palette.setColor(QPalette.Text, Qt.white)
    dark_palette.setColor(QPalette.Button, QColor(53, 53, 53))
    dark_palette.setColor(QPalette.ButtonText, Qt.white)
    dark_palette.setColor(QPalette.BrightText, Qt.red)
    dark_palette.setColor(QPalette.Link, QColor(42, 130, 218))
    dark_palette.setColor(QPalette.Highlight, QColor(42, 130, 218))
    dark_palette.setColor(QPalette.HighlightedText, Qt.black)
    app.setPalette(dark_palette)
    app.setStyleSheet("QToolTip { color: #ffffff; background-color: #2a82da; border: 1px solid white; }")

    layout = QHBoxLayout()
    button_ok = QPushButton('OK')

    button_exit = QPushButton('Exit')
    layout.addWidget(button_ok)
    layout.addWidget(button_exit)
    mywindow.setLayout(layout)
    mywindow.show()

    def exitButton():
        app.exit(0)

    button_exit.clicked.connect(exitButton)

    app.exec_()