win: use modern API

Zhao Yuhang

2023-12-22 adb46e1e151b73e56a7e0066a896b48b92a7230e

 examples/mainwindow/dark-style.qss

@@ -142,12 +142,12 @@

/* Window */

MainWindow[custom-style=true] {
    background-color: transparent;
MainWindow {
    background-color: #1E1E1E;
}

MainWindow[custom-style=false] {
    background-color: #1E1E1E;
MainWindow[custom-style=true] {
    background-color: transparent;
}

QWidget#clock-widget {