syncthingtray/tray/style.css

QtGui--TrayMenu {
    border: none;
    margin: 0px;
    padding: 0px;
}

QtGui--TrayWidget * {
    background-color: #333;
    color: white;
}

QtGui--TrayWidget Dialogs--SettingsDialog * {
    color: palette(WindowText);
    background-color: palette(base);
}