在组件中绑定官方提供的属性：popper-class，选择器下拉菜单的自定义类名

<el-select v-model="currentYear" @visible-change="yearChange($event)" popper-class="date_popper">
       <el-option v-for="item in years" :key="item.value" :label="item.label + ' 年'" :value="item.value">
       </el-option>
</el-select>

再添加一个标签<style lang="less"></style>，样式写在这个标签里即可

<style lang="less">
//下拉框背景色
.el-popper.is-light.date_popper {
    color: #000 !important;
    background: rgba(255, 255, 255, 0.68);
    border: 1px solid #3782b4 !important;
    backdrop-filter: blur(4px);
    -webkit-backdrop-filter: blur(4px);
}

//下拉框的链接小方块
.date_popper.el-popper[data-popper-placement^=bottom] .el-popper__arrow::before {
    background: rgba(0, 136, 255, 0.767) !important;
    border: 1px solid rgba(255, 255, 255, 0.788) !important;
}

.date_popper.el-popper[data-popper-placement^=top] .el-popper__arrow::before {
    border: 1px solid #254277 !important;
    background-image: linear-gradient(to right, #B6FAEB, #65ECC1);
}

//鼠标移动上去的选中色
.date_popper {

    .el-select-dropdown__item.hover,
    .el-select-dropdown__item:hover {
        background-image: linear-gradient(to right, #B6FAEB, #65ECC1);
    }

    //下拉框的文本颜色
    .el-select-dropdown__item {
        color: #39a0e6 !important;
    }

    //选中之后的颜色
    .el-select-dropdown__item.selected {
        background-image: linear-gradient(to right, #B6FAEB, #65ECC1);
    }
}
</style>