#26 Fail to compile with QT5.15

[clas atas]

qgtkstyle.cpp: In member function ‘virtual void QGtkStyle::polish(QApplication*)’:
qgtkstyle.cpp:443:30: error: ‘setSystemPalette’ is not a member of ‘QApplicationPrivate’
443 | QApplicationPrivate::setSystemPalette(standardPalette());
| ^~~~~~~~~~~~~~~~
make[4]: *** [Makefile:559: .obj/qgtkstyle.o] Error 1
make[4]: *** Waiting for unfinished jobs....
qgtkstyle_p.cpp: In member function ‘void QGtkStyleUpdateScheduler::updateTheme()’:
qgtkstyle_p.cpp:511:32: error: ‘setSystemPalette’ is not a member of ‘QApplicationPrivate’
511 | QApplicationPrivate::setSystemPalette(newPalette);
| ^~~~~~~~~~~~~~~~