This is a complete (I think) dark color palette for the Qt 5 Fusion theme, as well as a nice style sheet for the tool tips that make them blend better with the rest of the theme. To have immediate effect, be sure to put this in your main function before showing the parent window. Child windows should automatically inherit the palette unless you …

Qt 5 Dark Fusion Palette

    qApp->setStyle(QStyleFactory::create("Fusion"));

    QPalette darkPalette;
    darkPalette.setColor(QPalette::Window, QColor(53,53,53));
    darkPalette.setColor(QPalette::WindowText, Qt::white);
    darkPalette.setColor(QPalette::Base, QColor(25,25,25));
    darkPalette.setColor(QPalette::AlternateBase, QColor(53,53,53));
    darkPalette.setColor(QPalette::ToolTipBase, Qt::white);
    darkPalette.setColor(QPalette::ToolTipText, Qt::white);
    darkPalette.setColor(QPalette::Text, Qt::white);
    darkPalette.setColor(QPalette::Button, QColor(53,53,53));
    darkPalette.setColor(QPalette::ButtonText, Qt::white);
    darkPalette.setColor(QPalette::BrightText, Qt::red);
    darkPalette.setColor(QPalette::Link, QColor(42, 130, 218));

    darkPalette.setColor(QPalette::Highlight, QColor(42, 130, 218));
    darkPalette.setColor(QPalette::HighlightedText, Qt::black);
    
    qApp->setPalette(darkPalette);

    qApp->setStyleSheet("QToolTip { color: #ffffff; background-color: #2a82da; border: 1px solid white; }");

When I had floating docks using this I got white lines on the left edge.
Setting QPallete::Light to the same color as QPalette::Window fixed it.

darkPalette.setColor(QPalette::Light,QColor(53, 53, 53));