Before we start I suggest you take a look at this post, we'll be using that same approach but with React.

---

Let us get started with a simple React component called App:

import React from 'react'

import './styles.css'

function App() {
  return (
    <div className="App">
      <h1>Hello stranger!</h1>
      <h2>Lets persist our dark theme, shall we?</h2>
    </div>
  )
}

export default App

Now lets use useState to set our default value for darkMode, which will be false:

import React, { useState } from "react";

import "./styles.css";

function App() {
  const [darkMode, setDarkMode] = useState(false);

  // ...

Now, lets change our div to include a data-theme attribute, we'll check if the value of darkMode is true and set a value of dark, otherwise it's light:

// ...

function App() {
  return (
    <div className="App" data-theme={darkMode ? 'dark' : 'light'}>
      <h1>Hello stranger!</h1>
      <h2>Lets persist our dark theme, shall we?</h2>
    </div>
  )
}

// ...

Now if you change our useState() from false to true you should see that it sets the background of .App to black and the text color to white.

---

Switch

Let us build a switch between dark and light mode. We can start by adding a new function that sets our theme either to light or to dark mode:

import React, { useState } from "react";

import "./styles.css";

function App() {
  const [darkMode, setDarkMode] = useState(false);
  const toggleDarkMode = () => setDarkMode(darkMode ? false : true);

  // ...

We can still use a little trick on darkMode ? false : true, we can change it to !darkMode, what does this means?

Since darkMode is a boolean we're saying we want the opposite value, see the output:

var darkMode = true

darkMode = !darkMode // false
darkMode = !darkMode // true
darkMode = !darkMode // false
darkMode = !darkMode // true

We can do this all day!

---

Button Switcher

Now that we have our toggleDarkMode function in place let us create a <button> to use it:

function App() {
  return (
    <div className="App" data-theme={darkMode ? 'dark' : 'light'}>
      <button onClick={toggleDarkMode}>Dark mode toggler</button>
      <h1>Hello stranger!</h1>
      <h2>Lets persist our dark theme, shall we?</h2>
    </div>
  )
}

If you try it out is should work just fine, switching from light to dark mode and vice-versa. We can still use our darkMode state to manipulate what the button displays:

<button onClick={toggleDarkMode}>
  {darkMode ? 'Lights ON!' : 'Lights OFF!'}
</button>

Let us move on.

---

Persist

This is all good, but what if you switch to dark mode and refresh the page? We're back to light mode. Let us fix that shall we?

For that we'll be using React's useEffect and localStorage. First we need to import:

import React, { useState, useEffect } from 'react'

With useEffect we can run some code every time a value changes, in this case we want to run our code when darkMode changes:

import React, { useState, useEffect } from "react";

import "./styles.css";

function App() {
  const [darkMode, setDarkMode] = useState(false);
  const toggleDarkMode = () => setDarkMode(darkMode ? false : true);

  useEffect(() => {
    console.log(`Is in dark mode? ${darkMode}`);
  }, [darkMode]);

  // ...

If you check the console you'll see what's happening every time we toggle dark mode. Now that we know this is working we can store our value in localStorage:

useEffect(() => {
  console.log(`Is in dark mode? ${darkMode}`)
  localStorage.setItem('DARK_MODE', darkMode)
}, [darkMode])

Alright, all good so far, now we just need read the value of DARK_MODE from localStorage and set our default darkMode value:

const storedDarkMode = localStorage.getItem('DARK_MODE')
const [darkMode, setDarkMode] = useState(storedDarkMode)

Give it a try, change to dark mode and then refresh your page. Boom 💥

That's all, hope it helped you in some way.