body{
    background-color: red;
}

.telo{
    margin-left: 3rem;
}

.main{
    display:flex;
}

.header{
    position: sticky;
    top: 0;
    z-index: 1;
    font-size: 2em;
    margin-bottom: 10px;
}

.cntnr{
    background-color: #eeeeee;
    margin-bottom: 10px;
}

.bigbox{
    height: 800px;
    width: 80%;
    background-color: lightyellow;
    font-size: 1.6rem;
    padding-left: 1rem;
}

   @media screen and (max-width: 600px) {
     body { background: #fc0; }
   }

@media screen and (max-width: 800px){
    .cntnr{
        background-color: magenta;
    }
}