> m_newTabAction->setIcon(QIcon(QLatin1String(":graphics/addtab.png")));First guess: The Breeze icon engine (which enables color schemes to recolor monochrome icons) can not handle Qt ressource paths. Did you try to install the image as ordinary file, and use that file as icon? If that is the problem, a workaround could be to load the ressource through QPixmap instead of QIcon. Or are you sure that the icon is loaded correctly, and just the QToolButton does not show it?
