Simple theme icon switching using the QIconEgine

icon_engine.py

"Based on https://stackoverflow.com/a/76096769"
import sys
from PyQt6.QtWidgets import QApplication, QWidget, QToolButton, QComboBox, QVBoxLayout, QHBoxLayout
from PyQt6.QtGui import QIcon, QIconEngine, QPainter
from PyQt6.QtCore import QSize, QRect

themes = ['light', 'dark']
active_theme = themes[0]

light = '''
* {
  font-family:"Helvetica Neue", Helvetica, Arial;
  font-size: 12px;
  color: #333;
  background-color: rgb(240, 240, 240);
}
'''

dark = '''
*   {
        font-family:"Helvetica Neue", Helvetica, Arial;
        font-size: 12px;
        color: rgb(255, 255, 255);
        background-color: rgb(52, 49, 49);
    }
'''


def get_theme_path():
    # modify this to have the light and dark folder
    if active_theme == themes[1]:
        return 'dark/'
    return '' 


class IconEngine(QIconEngine):

    def __init__(self, file_name):
        super().__init__()
        self.theme = active_theme
        self.icon = QIcon(file_name)
        self.file_name = file_name

    def updateIcon(self):
        if self.theme != active_theme:
            # path should be created with pathlib but this is quick
            path = f'{get_theme_path()}{self.file_name}'
            self.icon = QIcon(path)
            self.theme = active_theme

    def pixmap(self, size, mode, state):
        self.updateIcon()
        return self.icon.pixmap(size, mode, state)

    def paint(self, painter, rect, mode, state):
        self.updateIcon()
        return self.icon.pixmap.paint(painter, rect, mode, state)

    
def changeStyle(name):
    global active_theme
    if name == themes[0]:
        QApplication.instance().setStyleSheet(light)
    else:
        QApplication.instance().setStyleSheet(dark)
    active_theme = name


class Window(QWidget):
    def __init__(self):
        super().__init__()

        but1 = QToolButton()
        but2 = QToolButton()

        icon1 = QIcon(IconEngine('unchecked.png'))
        icon2 = QIcon(IconEngine('checked.png'))

        but1.setIcon(icon1)
        but2.setIcon(icon2)

        cmb = QComboBox()
        cmb.addItems(themes)
        cmb.currentTextChanged.connect(changeStyle)
        changeStyle(themes[0])

        tbs = QHBoxLayout()
        tbs.addWidget(but1)
        tbs.addWidget(but2)
        lay = QVBoxLayout(self)
        lay.addWidget(cmb)
        lay.addLayout(tbs)

app = QApplication([])
window = Window()
window.show()
sys.exit(app.exec())