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App.vue
main.js
 
<template>
  <div ref="rootDiv">
    <h2>Example $el Object</h2>
    <p>It is recommended, and more consistent, to use the ref attribute instead of the $el object to change the background color root DIV tag.</p>
    <button v-on:click="changeColor">Click here</button>
  </div>
</template>

<script>
export default {
  methods: {
    changeColor() {
      this.$refs.rootDiv.style.backgroundColor = 'lightgreen';
    }
  }
}
</script>                  
import { createApp } from 'vue'

import App from './App.vue'

const app = createApp(App)
app.mount('#app')
                  
http://localhost:5173/