Thank you @chehrlic for a solution that worked.

Using on windows, adding the preprocessor OS check for windows and changing the style to 'windowsvista' if true, solved the immediate problem.

main.c

#include <QStyleFactory>

// if windows, set this style
#ifdef Q_OS_WIN
    if (QStyleFactory::keys().contains("windowsvista")) {
        a.setStyle(QStyleFactory::create("windowsvista"));
    }
#endif