【theme】about theme-override in Config Provider

[xxholly32]

能够在全局设置替换皮肤的参数，参考了下 naive-ui 是在 Config Provider 引入了 theme-override 属性去控制的；由于element-plus 暂时先分成全局的变量（在root中设置），以及组件变量（在组件css中设置的），那么提案大概是这样的

<template>
  <el-config-provider :theme-override="theme">
    <el-button>default</el-button>
  </el-config-provider>
</template>

<script setup lang="ts">
import { ref } from 'vue'

const theme = ref({
  common: {
    "--el-fill-color-blank": '#ffcccc',
  }
  // button: {
  //   ...
  // }
})
</script>

对于换肤还想做的事情有些多，比如类似 #7955 (comment) 以及组件的换肤功能，我想在这个功能实现以后再提~，希望官方可以考虑这个提案，我也会提交pr

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Set the parameters of the replacement skin globally. See naive-ui, which introduces the theme-override attribute in Config Provider for control. Since element-plus is temporarily divided into global variables (set in root) and component variables (set in component css), the proposal looks something like this

<template>
  <el-config-provider :theme-override="theme">
    <el-button>default</el-button>
  </el-config-provider>
</template>

<script setup lang="ts">
import { ref } from 'vue'

const theme = ref({
  common: {
    "--el-fill-color-blank": '#ffcccc',
  }
  // button: {
  //   ...
  // }
})
</script>

---

To also want to do some more, such as such as #7955 (comment) and component change skin function, I would like to mention this function after the implementation of ~, I hope the official can consider this proposal, I will also submit pr