body {
    background-color: darkslategray;
}

.zahlavi {
    width: 100%;
    background-color: lightseagreen;
    height: 50px;
    text-align: center;
    font-size: 40px;
    margin-left: auto;
    margin-right: auto;
}

.clanek {
    box-sizing: border-box;
    width: 100%;
    height: 600px;
    margin: 20px 0;
    padding: 15px;
    background-color: black;
}

@media screen and (min-width: 900px) {
    .clanek {
        width: 79%;
        float: left;
    }
}


.postrannipanel {
    box-sizing: border-box;
    width: 100%;
    height: 400px;
    margin: 20px 0;
    padding: 10px;
    background-color: cadetblue;
}

@media screen and (min-width: 900px) {
    .postrannipanel {
        width: 21%;
        float: right;
    }
}


.panel1 {
    text-align: center;
    background-color: gray;
    height: 300px;
}

.panel2 {
    text-align: center;
    background-color:slategray;
}