section.timeline::after{
    content: "";
    position: absolute;
    left: calc(50% - 2px);
    width: 4px;
    top: 0;
    bottom: 0;
    opacity: .2;
    background: linear-gradient(180deg, #1990EA 0%, #48c81a 25%, #ff8c00 50%, #ff0000 75%, transparent 100%);
}

section.timeline > div > .row-container::after{
    border: 4px solid #fff;
    position: absolute;
    left: calc(50% - 17px);
    width: 34px;
    height: 34px;
    border-radius: 34px;
    top: -5px;
    bottom: 0;
    text-align: center;
    line-height: 1.5;
    font-size: 18px;
    z-index: 1;
    font-weight: bold;
    color: #fff;
    box-shadow: 0 0 4px rgba(0,0,0,.2);
}

section.timeline #row-1::after{
    content: "1";
    background: #1990EA;
}

section.timeline #row-2::after{
    content: "2";
    background: #48c919;
}

section.timeline #row-3::after{
    content: "3";
    background: darkorange;
}

section.timeline #row-4::after{
    content: "4";
    background: red;
}