div#rainbow-block {
    height: 100px;
    width: 100%;
    background: linear-gradient(to right, red, orange, yellow, green, cyan, blue, purple);
}

div#horn-block {
    width: 0;
    height: 0;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 100px solid darkblue;
    margin-left: 100px;
    margin-top: -40px;
}