.zahlavi

{
    width: 100%;
    background-color: azure;
    height: 150px;
    text-align: center;
    font-size: 40px
}

.clanek {
    box-sizing: border-box;
    width: 100%;
    height: 500px;
    margin: 20px 0;
    padding: 15px;
    background-color: antiquewhite;
}

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

.postranni-panel {
    box-sizing: border-box;
    width: 100%;
    height: 200px;
    margin: 20px 0;
    padding: 10px;
    background-color: #F7B492;
}

@media screen and (min-width: 900px) {
    .postranni-panel {
        width: 19%;
        float: right
    }
}

.panel_1 {
    text-align: center;
    background-color: gainsboro
}

.panel_2 {
    text-align: center;
    background-color: mediumspringgreen
}