引申:
网上的方式大多是通过在静态文件夹下存放两套CSS,
在index.html里,
动态写入 link标签,
动态切换CSS,
这种方法理论来说可行,但是可能存在样式覆盖问题,

后来尝试后不可行,于是把主意打到了main.js里面

const styleDefine = localStorage.get('GLOBAL_THEME')//缓存的变量
let loadStyle
async function importCss() {
  if (styleDefine === 'dark') {
    loadStyle = await import('./assets/XXXX1.css')
  } else {
    loadStyle = await import('./assets/XXXX2.css')
  }
  return loadStyle
}
importCss()

这种方法通过你的缓存来切换主题,但是主题是切换了,但是需要手动刷新一下,

综上所述 :
触发:

changeCss(event) {
      if (event === '1') {
        this.$store.dispatch('setThemeFun', 'dark')
        localStorage.set(GLOBAL_THEME, 'dark')
      } else {
        this.$store.dispatch('setThemeFun', 'light')
        localStorage.set(GLOBAL_THEME, 'light')
      }
    }

Vuex:

changeGlobalTheme: (state, value) => {
      state.GlobalTheme = value
      if (storage.get(GLOBAL_THEME) !== value) {
        window.location.reload()  //手动刷新才能生效
      }
      localStorage.set(GLOBAL_THEME, value)
      state.theme = value
    }

main.js

const styleDefine = localStorage.get('GLOBAL_THEME')//缓存的变量
let loadStyle
async function importCss() {
  if (styleDefine === 'dark') {
    loadStyle = await import('./assets/XXXX1.css')
  } else {
    loadStyle = await import('./assets/XXXX2.css')
  }
  return loadStyle
}
importCss()