body {
    background: lightblue; /* Цвет фона веб-страницы */
}

.maxwidth {
    position: absolute;
    background: white;
    border: 0px solid rgb(100, 100, 100);
    left: 0%;    top: 0px;
    width: 100%; height: 100%;
}

/* =============== MOBILE ====================== */
.side-left {                                   /* mobile */
    display: none;
}

.indice {                                     /* mobile */
    display: none;
}

.content_corso {                             /* mobile */
    display: block;
    background:  rgb(210, 235, 210);
    border: 0px solid blue;
    position: absolute;
    left:  0%;   top:0.0%;
    width: 100.0%;   height: 100%;
    padding-left:0%; padding-top: 0%;
    color: black; font-size: 2.1vh;
}


.side-right {                                /* mobile */
    display: none;
}


<!--   =================  End MEDIA Mobile ======================  -->

<!--   =================== MEDIA COMPUTER ======================  -->

body {  }

@media (min-width: 450px) {

.maxwidth {                              /* computer */
    position: absolute;
    background: white;
    border: 0px solid rgb(50, 50, 50);
    left: 0%;    top:0.0%;
    width: 100%; height: 100%;
    padding-top: 0%;
    margin: 0;
}

.side-left {                               /* computer */
    display: block;
    background:  rgb(70, 70, 70);
    background:  darkgreen;
    border: 0px solid red;
    position: absolute;
    left: 0.0%;     top:0.0%;
    width: 17.76%;   height: 99.9%;
    padding-left:0%; padding-top: 0%;
    color: black; font-size: 1.1vw;
}
.indice {                                 /* computer */
    display: block;
    background:  rgb(180, 255, 180);
    border: 0px solid brown;
    position: absolute;
    left: 18.0%;     top:0.0%;
    width: 18.9%;   height: 99.9%;
    padding-left:0%; padding-top: 0%;
    color: black; font-size: 1.1vh;
}
.content_corso {                                      /* computer */
    display: block;
    background:  rgb(210, 235, 210);
    border: 0px solid blue;
    position: absolute;
    left:  37.1%;   top:0.0%;
    width: 45.0%;   height: 99.9%;
    padding-left:0%; padding-top: 0%;
    color: black; font-size: 1.1vh;
}

.side-right {                               /* computer */
    display: block;
    background:  rgb(70, 70, 70);
    background:  darkgreen;
    border: 0px solid black;
    position: absolute;
    left: 82.3%;     top:0.0%;
    width: 17.7%;   height: 99.9%;
    padding-left: 0;
    font-size: 1.1vh;
}


}
 <!--   =================== END of MEDIA COMPUTER ======================  -->