index.css

.main {
  margin-left: 80px
}
#nav {
    height: 100%;
    width: 80px;
    position: fixed;
    top: 0;
    left: 0;
    background-color: #263238;
    overflow-x: hidden;
    padding-top: 10%;
}

#nav a {
    display: block;
    height: 80px;
    width: 80px;
}

#nav div {
    display: block;
    height: 80px;
    width: 80px;
    background-size: 60px 60px;
    background-repeat: no-repeat;
    background-position: center;
}

.menu-atom {
    background-color: #0d47a1
}

.atom {
    background: url(atom.svg);
}

.menu-flower {
    background-color: #1b5e20
}

.flower {
    background: url(flower.svg);
}