in qquickstyle.cpp:

        const QByteArray value = qgetenv("QT_QUICK_CONTROLS_STYLE_PATH");
        paths += QString::fromLocal8Bit(value).split(QDir::listSeparator(), QString::SkipEmptyParts);

On linux/macOS, this will split on color character which is also used to mark resource paths, so it is impossible on these platforms to point to locations in resources (eg. :/mystyles) though QT_QUICK_CONTROLS_STYLE_PATH environment variable.