website/nav.css

body {
    margin: 0;
}

body > * {
    box-sizing: border-box;
    float: left;
    padding: 4px;
}

#main {
    /*position: absolute;
    top: 0;
    left: 192px;*/
    width: calc(100% - 192px);
}

#nav {
    width: 192px;
    background-color: #cccccc;
    /*position: fixed;
    top: 0;
    left: 0;*/
}

.nav-item {
    padding: 0px 4px 0px 4px;
    margin: 2px;
    background-color: #800000;
    color: #ddd;
    display: block;
}

.nav-item:hover {
    background-color: #ffffff;
    color: #800000;
}

.nav {
    width: 100%;
}

@media only screen and (min-width: 576px) {
    #nav {
        position: fixed;
        top: 0;
        left: 0;
    }
    #main {
        position: absolute;
        top: 0;
        left: 192px;
    }
}

@media only screen and (max-width: 576px) {
    #main, #nav {
        width: 100%;
    }
}