.knop_index {
    margin-left: auto;
    margin-right: auto;
    margin-top: 3vw;
    display: block;
    width: 70vw;
    padding: 1vw;
    font-size: 4.8vw;
    background-color: #eee;
    color: #555;
    border: 2px solid #fff;
    border-radius: 5%;
    }
.knop_index:hover {
    background-color: #fff;
    border: 2px solid #da5;
    }

@media only screen and (orientation: landscape) {
.knop_index {
    margin-left: auto;
    margin-right: auto;
    margin-top: 2vw;
    display: block;
    width: 60vw;
    padding: 1vw;
    font-size: 3.9vw;
    background-color: #eee;
    color: #555;
    border: 2px solid #fff;
    border-radius: 5%;
    }
}

@media only screen and (orientation: landscape) and (min-width: 750px) {
.knop_index {
    display: inline-block;
    width: 36vw;
    font-size: 2.8vw;
    }
}