.arrow {
    position: relative;
}

/* Inner triangle (should be invisible and blended in with the background */
.arrow:after {
    border-width: 5px;
    border-left-color: #ffffff; /* The color should be the same as the background to let the triangle blend in with the background */
    margin-top: 4px;
    margin-left: -6px;
}

/* triangle border */
.arrow:before {
    border-width: 6px;  /* when increasing the border thickness, you should also INCREASE the "margin-top" and DECREASE the "margin-left" attributes of the ":after" css */
    border-left-color: #666666;
    margin-left: -5px;
    margin-top: 3px;
}