<template>
  <header>
    <div class="left">left</div>
    <div class="center">center</div>
    <div class="right">right</div>
  </header>
  <div>
    <button @click="changeSize(36)">大（36）</button>
    <button @click="changeSize(24)">中（24）</button>
    <button @click="changeSize(12)">?。?2）</button>
  </div>
</template>

<script setup lang="ts">
import { useCssVar } from '@vueuse/core'
const changeSize = (number:number) => {
  const size = useCssVar('--size')
  size.value = number + 'px'
}
</script>

<style scoped lang="less">
:root {
  --size: 12px;
}
header {
  display: flex;

  .left {
    width: 100px;
    height: 50px;
    font-size:  var(--size);
    background: lightblue;
  }

  .center {
    flex: 1;
    height: 50px;
    font-size:  var(--size);
    background: lightcoral;
  }

  .right {
    width: 100px;
    height: 50px;
    font-size:  var(--size);
    background: lightgoldenrodyellow;
  }
}
</style>

const changeSize = (number:number) => {
  document.documentElement.style.setProperty('--size',number + 'px')
  // document.documentElement.style.getPropertyValue('--size')
  // const size = useCssVar('--size')
  // size.value = number + 'px'
}