.additionalnavi.mainnav {
    display: inline-block;
}
.mainnavigation {
    background-color: #fff;
    display: none;
    position: fixed;
}

.bggrey {
    background-color: #000;
    display: none;
    height: 100%;
    opacity: 0.8;
    position: fixed;
    width: 100%;
    top: 0;
}

@media (min-width: 540px) {
    .additionalnavi.mainnav {
        display: none;
    }
    .bggrey {
        display: none;
    }
    .mainnavigation {
        display: block;
        position: relative;
    }
}