fix compilation error for old qt versions

Zhao Yuhang

2025-02-09 bc03db3759e0d8644e626adcbda066ccb2c30062

 examples/mainwindow/dark-style.qss

@@ -1,11 +1,13 @@
/* Window bar */

QWK--WindowBar[bar-active=true] {
    background-color: #3C3C3C;
    /*background-color: #3C3C3C;*/
    background-color: transparent;
}

QWK--WindowBar[bar-active=false] {
    background-color: #505050;
    /*background-color: #505050;*/
    background-color: transparent;
}