SO-45545694

https://stackoverflow.com/questions/45545694/how-to-modify-the-border-bottom-color-of-pseudo-class-elements-through-javascr

by Siddiqui Noor

HTML

<div id="calendar" class="calendar-content">

</div>

CSS

.calendar-content .canChoose {
    color: #4A494A;
    font-size: 14px;
    font-weight: bold;
    border-bottom: 1px solid #BBBBBB;
}

.calendar-content .canChoose:after {
    width: 20px;
    margin-top: 5px;
    margin-left: auto;
    margin-right: auto;
    border-bottom: 8px solid #5A8C19; /*Change the color here*/
    border-radius: 10px;
    content: "";
    display: block;
}

JavaScript

var monDaynum = 5;	// temp data for test
var nowDay = 3;			// temp data for test

var el = document.getElementById('calendar'); // temp DOM to test

for(var i = 1; i <= monDaynum; i++) {
    var subContent = document.createElement("div");
    subContent.className = "canChoose";
    if(nowDay === i){
        subContent.style.borderBottomColor = "red" //It works
    } else {

    }
    if(i == nowDay){
        subContent.classList.add("today");
    }
    subContent.innerHTML = i;
    el.appendChild(subContent);
}