基于CSDN的三步走，到哪步能用了就停手：

第一步：在`el-select`添加属性`mySelectStyle`（名称可自定义）。详细连接如下：

Vue3：修改下拉框el-select的样式-CSDN博客

懒得跳转过去的朋友直接看

代码摘要：

<el-select 
v-model="value" 
placeholder="请选择" 

popper-class="mySelectStyle">
//popper-class='内容可自定义'
    <el-option />
</el-select>

---

第二步：在`style`中使用`::deep`或`/deep/`，结果发现不生效。详细连接如下：

Vue中的Element-plus的Select选择器下拉框样式如果自定义修改（popper-append-to-body(已废弃)的平替popper-class属性的解决方法）-CSDN博客

【注意：此代码为我这不生效的代码】 

代码摘要

<style lang="less">
//下拉框标题文本label
::v-deep .el-form-item__label{
  color: #fff;
  font-weight: 100;
}
//选择框
::v-deep .el-input__wrapper{
  background-color: rgba(26, 84, 128,0.5);
  width:130px;
}
//下拉框文字
/deep/ .el-input__inner{
 color:#e1dcdc;
}


//下拉框背景色
.el-popper.is-light.mySelectStyle {
    background-color: rgba(0, 136, 255, 0.1) !important;
    border: 1px solid #254277 !important;
}
 //下拉框的链接小方块
.mySelectStyle.el-popper[data-popper-placement^=bottom] .el-popper__arrow::before {
    background: rgba(0, 136, 255, 0.1) !important;
    border: 1px solid #254277 !important;
}
.mySelectStyle.el-popper[data-popper-placement^=top] .el-popper__arrow::before {
    background: #254277 !important;
    border: 1px solid #254277 !important;
}
 //鼠标移动上去的选中色
.mySelectStyle {
    .el-select-dropdown__item.hover,
    .el-select-dropdown__item:hover {
        background: #2A77C6 !important;
    }
 //下拉框的文本颜色
    .el-select-dropdown__item {
        color: #B3BCCE  !important;
    }
 //选中之后的颜色
    .el-select-dropdown__item.selected {
        background: #2A77C6 !important;
    }
}

</style>

 效果如图

---

第三步：通过不同的style，解决了不生效的情况，但是不确定是否会与其他样式冲突。

详细链接如下：

Vue3中样式渗透:deep()为什么无效_vue3 deep-CSDN博客 

将deep单独写在一个style内并加上scoped，成功的生效了

代码摘要：

<style lang="less" scoped>
:deep(.el-input__wrapper) {
    background: linear-gradient(90deg, #0b3571, rgba(11, 53, 113, 0) 100%);
    box-shadow: 0 0 0 0 var(--el-input-border-color, var(--el-border-color)) inset;
}

:deep(.el-select:hover:not(.el-select--disabled) .el-input__wrapper) {
    box-shadow: 0 0 0 0 var(--el-select-input-focus-border-color) inset !important;
}

:deep(.el-select .el-input.is-focus .el-input__wrapper) {
    box-shadow: 0 0 0 0 var(--el-select-input-focus-border-color) inset !important;
}

:deep(.el-select:hover:not(.el-select--disabled) .el-input__wrapper) {
    background: #0b3571;
}

:deep(.el-select .el-input__wrapper) {
    background: #0b3571;
}
</style>

<style lang="less">
//下拉框的背景色
.el-select-dropdown__item {
    background: #254277 !important;
}

//下拉框的链接小方块
.el-select-dropdown__item.is-active {
    background: #2A77C6 !important;
}

//鼠标移动上去的选中色
.el-select-dropdown__item.hover,
//下拉框背景色
.el-popper.is-light.mySelectStyle {
    background-color: rgba(0, 136, 255, 0.1) !important;
    border: 1px solid #254277 !important;
}

//下拉框的链接小方块
.mySelectStyle.el-popper[data-popper-placement^=bottom] .el-popper__arrow::before {
    background: rgba(0, 136, 255, 0.1) !important;
    border: 1px solid #254277 !important;
}

.mySelectStyle.el-popper[data-popper-placement^=top] .el-popper__arrow::before {
    background: #254277 !important;
    border: 1px solid #254277 !important;
}

//鼠标移动上去的选中色
.mySelectStyle {

    .el-select-dropdown__item.hover,
    .el-select-dropdown__item:hover {
        background: #2A77C6 !important;
    }

    //下拉框的文本颜色
    .el-select-dropdown__item {
        color: #B3BCCE !important;
    }

    //选中之后的颜色
    .el-select-dropdown__item.selected {
        background: #2A77C6 !important;
    }
}
</style>

效果如图

 

---

完整代码，可直接复制为组件使用（需全局安装element-plus）：

<template>
    <el-form-item label="统计频度">
        <el-select v-model="value" value-key="id" placeholder="请选择" popper-class="mySelectStyle" clearable filterable>
            <el-option v-for="item in option" :key="item.id" :label="item.label" :value="item.value"
                :disabled="item.disabled" />
        </el-select>
    </el-form-item>
</template>

<script lang="ts">
import { ref, reactive, defineComponent } from 'vue'
export default defineComponent({
    name: 'MySelect',
    props: {
        value: {
            type: String,
            default: ''
        },
        option: {
            type: Array,
            default: () => []
        }
    },
    setup(props) {
        const value = ref(props.value)
        const option = reactive(props.option)
        return {
            value,
            option
        }
    }
})
</script>

<style lang="less" scoped>
:deep(.el-input__wrapper) {
    background: linear-gradient(90deg, #0b3571, rgba(11, 53, 113, 0) 100%);
    box-shadow: 0 0 0 0 var(--el-input-border-color, var(--el-border-color)) inset;
}

:deep(.el-select:hover:not(.el-select--disabled) .el-input__wrapper) {
    box-shadow: 0 0 0 0 var(--el-select-input-focus-border-color) inset !important;
}

:deep(.el-select .el-input.is-focus .el-input__wrapper) {
    box-shadow: 0 0 0 0 var(--el-select-input-focus-border-color) inset !important;
}

:deep(.el-select:hover:not(.el-select--disabled) .el-input__wrapper) {
    background: #0b3571;
}

:deep(.el-select .el-input__wrapper) {
    background: #0b3571;
}
</style>

<style lang="less">
//下拉框的背景色
.el-select-dropdown__item {
    background: #254277 !important;
}

//下拉框的链接小方块
.el-select-dropdown__item.is-active {
    background: #2A77C6 !important;
}

//鼠标移动上去的选中色
.el-select-dropdown__item.hover,
//下拉框背景色
.el-popper.is-light.mySelectStyle {
    background-color: rgba(0, 136, 255, 0.1) !important;
    border: 1px solid #254277 !important;
}

//下拉框的链接小方块
.mySelectStyle.el-popper[data-popper-placement^=bottom] .el-popper__arrow::before {
    background: rgba(0, 136, 255, 0.1) !important;
    border: 1px solid #254277 !important;
}

.mySelectStyle.el-popper[data-popper-placement^=top] .el-popper__arrow::before {
    background: #254277 !important;
    border: 1px solid #254277 !important;
}

//鼠标移动上去的选中色
.mySelectStyle {

    .el-select-dropdown__item.hover,
    .el-select-dropdown__item:hover {
        background: #2A77C6 !important;
    }

    //下拉框的文本颜色
    .el-select-dropdown__item {
        color: #B3BCCE !important;
    }

    //选中之后的颜色
    .el-select-dropdown__item.selected {
        background: #2A77C6 !important;
    }
}
</style>