I found the following description:

[virtual] QPalette QStyle::standardPalette() const
Returns the style's standard palette.
Note that on systems that support system colors, the style's standard palette is not used. In particular, the Windows XP, Vista, and Mac styles do not use the standard palette, but make use of native theme engines. With these styles, you should not set the palette with QApplication::setStandardPalette().

But QApplication::setStandardPalette() is missing. Please, fix it.