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<style>
[data-v-5f487179] .van-dropdown-menu__title::after {
    filter: brightness(75%);
    width: 0.173333rem;
    height: 0.173333rem;
}
<style>
.van-dropdown-menu__title::after {
    position: absolute;
    top: 50%;
    right: -0.106667rem;
    margin-top: -0.133333rem;
    border: 0.08rem solid;
    border-color: transparent transparent #dcdee0 #dcdee0;
    -webkit-transform: rotate(-45deg);
    transform: rotate(-45deg);
    opacity: .8;
    content: '';
}
<style>
*:before, *:after {
    box-sizing: border-box;
}