Prevent flicker when using dark mode

<script nonce="<add your generated guid here>">
  /**
   * A IIFE to set the theme mode for the website.
   * Helps prevent flickering effect (FOUC) when using dark themes and =
   * refreshing.
   **/
  ;(function themeSetter() {
    const key = "theme_mode"
    let currentThemeMode = window?.localStorage?.getItem(key)
    let isThemeModeLight = currentThemeMode === "light"
    let isThemeModeDark = currentThemeMode === "dark"
    let hasThemeMode = isThemeModeDark || isThemeModeLight
 
    if (!hasThemeMode) {
      currentThemeMode = "dark"
      window?.localStorage?.setItem(key, currentThemeMode)
    }
 
    document?.getElementsByTagName("html")[0].classList.add(currentThemeMode)
    document?.body?.classList?.add(currentThemeMode)
  })()
</script>

References