<style>:

      :root {
        --mode: 'unknown';
      }
      @media (prefers-color-scheme: dark) {
        :root {
            --mode: 'dark';
         }
       }
      @media (prefers-color-scheme: light) {
        :root {
          --mode: 'light';
        }
      }

<script>:

data() {
    return { mode: '' }
  },
created() {
    this.mode = getComputedStyle(document.documentElement)
      .getPropertyValue('--mode')
      .trim()
  },

參考：https://www.zhangxinxu.com/wordpress/2020/02/css-params-to-js/