三角形

    $top: -15px;
    $left: 50%;
    &:after {
        position: absolute;
        top: $top;
        left: $left;
        content: ' ';
        z-index: 1;
        border: 8px solid transparent;
        border-bottom: 8px solid #EBEEF5;
    }
    &:before {
        position: absolute;
        top: $top;
        left: $left;
        z-index: 2;
        content: ' ';
        border: 8px solid transparent;
        border-bottom: 8px solid white;
    }