方式一：

.dropdown-menu.show:before {
    width: 14px;
    height: 14px;
    position: absolute;
    background: #fff;
    top: -8px;
    left: 26px;
    padding: 0;
    content: '';
    border: 1px solid #ebeef5;
    border-right: none;
    border-bottom: none;
    transform: rotate(45deg);
}

方式二：

.dropdown-menu.show:before {
    box-sizing: content-box;
    width: 0px;
    height: 0px;
    position: absolute;
    top: -16px;
    left: 24px;
    padding: 0;
    border-bottom: 8px solid #fff;
    border-top: 8px solid transparent;
    border-left: 8px solid transparent;
    border-right: 8px solid transparent;
    display: block;
    content: '';
    z-index: 12;
}

.dropdown-menu.show:after {
    box-sizing: content-box;
    width: 0px;
    height: 0px;
    position: absolute;
    top: -18px;
    left: 23px;
    padding: 0;
    border-bottom: 9px solid #ebeef5;
    border-top: 9px solid transparent;
    border-left: 9px solid transparent;
    border-right: 9px solid transparent;
    display: block;
    content: '';
    z-index: 10
}