Perfect Next.js dark mode in 2 lines of code. Support System preference and any other theme with no flashing | BestofReactjs

<button x-cloak x-data="{scroll : false}" @scroll.window="document.documentElement.scrollTop > 20 ? scroll = true : scroll = false" x-show="scroll" @click="window.scrollTo({top: 0, behavior: 'smooth'})" type="button" data-mdb-ripple="true" data-mdb-ripple-color="light" class="fixed inline-block p-3 text-xs font-medium leading-tight text-white uppercase transition duration-150 ease-in-out bg-blue-600 rounded-full shadow-md hover:bg-blue-700 hover:shadow-lg focus:bg-blue-700 focus:shadow-lg focus:outline-none focus:ring-0 active:bg-blue-800 active:shadow-lg bottom-5 right-5" id="btn-back-to-top" x-transition.opacity>
        <svg xmlns="http://www.w3.org/2000/svg" class="w-4 h-4" viewBox="0 0 20 20" fill="currentColor">
            <path fill-rule="evenodd" d="M3.293 9.707a1 1 0 010-1.414l6-6a1 1 0 011.414 0l6 6a1 1 0 01-1.414 1.414L11 5.414V17a1 1 0 11-2 0V5.414L4.707 9.707a1 1 0 01-1.414 0z" clip-rule="evenodd" />
        </svg>
</button>

import { useTheme } from 'next-themes'

const ThemeChanger = () => {
  const [mounted, setMounted] = useState(false)
  const { theme, setTheme } = useTheme()

  // When mounted on client, now we can show the UI
  useEffect(() => setMounted(true), [])

  if (!mounted) return null

  return (
    <div>
      The current theme is: {theme}
      <button onClick={() => setTheme('light')}>Light Mode</button>
      <button onClick={() => setTheme('dark')}>Dark Mode</button>
    </div>
  )
}

// Button.jsx
import classnames from 'classnames';

function Button ({size, bgColor, textColor, children}) {
    return (
        <button className={classnames("bg-${bgColor} text-${textColor} font-bold py-2 px-4 rounded", {
    "text-xs": size === 'sm'
    "text-xl": size === 'lg',
    })}>
        {children}
    </button>
    )
};

export default Button;

export function TailwindIndicator() {
  if (process.env.NODE_ENV === "production") return null

  return (
    <div className="fixed bottom-1 left-1 z-50 flex h-6 w-6 items-center justify-center rounded-full bg-gray-800 p-3 font-mono text-xs text-white">
      <div className="block sm:hidden">xs</div>
      <div className="hidden sm:block md:hidden">sm</div>
      <div className="hidden md:block lg:hidden">md</div>
      <div className="hidden lg:block xl:hidden">lg</div>
      <div className="hidden xl:block 2xl:hidden">xl</div>
      <div className="hidden 2xl:block">2xl</div>
    </div>
  )
}