How to change color of QMainWindow borders and title bar?

Question

The QMainWindow below is assigned a dark-gray background-color using QSS. I would also like to change the color of the borders and the color of the title bar.

How to achieve control of the appearance of the QMainWindow borders and titlebar?

I would like to know how to change their colors and how to control the borders width and the title-bar's height.

from PyQt4.QtCore import *
from PyQt4.QtGui import *

appStyle="""
QMainWindow{
background-color: darkgray;
}
"""

class GUI(QMainWindow):
    def __init__(self):
        super(GUI, self).__init__()  
        self.setStyleSheet(appStyle)

if __name__ == '__main__': 
    if not QApplication.instance(): app=QApplication([])
    w=GUI() 
    w.setStyleSheet(appStyle)
    w.show() 
    w.raise_()

    sys.exit(app.exec_())

Answer 1

To my knowledge you cannot (on Windows maybe with some special OS dependent calls) modify the borders and header of a top-level widget (real window on the desktop) in Qt because these are delivered from the operating system.

You can however, set the widget frameless and add borders for yourself.

Example:

from PySide import QtGui, QtCore

app = QtGui.QApplication([])
app.setStyleSheet('QMainWindow{background-color: darkgray;border: 1px solid black;}')

w = QtGui.QMainWindow()
w.setWindowFlags(QtCore.Qt.FramelessWindowHint)
w.show()

app.exec_()

And looks like

You see that unfortunately the usually header bar vanishes, so no dragging, no resizing, no closing or minimizing. This then all must be implemented yourself. See for example How can I handle events in the titlebar and change its color etc ?.

Answer 2

This can be implemented in QT C++ by detecting current system theme from Windows registry and applying the Dark Window hints. A dark style in addition to this can make dark light window modes.

A sample project demonstrating this is shared here if it helps to adapt

https://envyen.com/posts/2021-10-24-QT-Windows-Dark-theme/