Cover image for Create A Dark/Light Mode Switch with CSS Variables

Create A Dark/Light Mode Switch with CSS Variables

ananyaneogi profile image Ananya Neogi ・4 min read

Giving your users a way to customise the interface to their preference is a huge win for user experience. Here we are going to provide the user with a simple switch to toggle between dark mode and light mode and we will also try to remember their preference for future visits.

Let's begin!

If you don't have a website of your own to which you wish to add this functionality, use this demo website to follow along.

Adding the CSS

We will be adding CSS custom properties also known as CSS variables, which we can reference and modify throughout the document. If you wish to read more about custom properties you can read on MDN.
Here's the tldr; version -

Custom properties (sometimes referred to as CSS variables or cascading variables) are entities defined by CSS authors that contain specific values to be reused throughout a document. They are set using custom property notation (e.g., --main-color: black;) and are accessed using the var() function (e.g., color: var(--main-color);)

First, we'll add our light or default mode css variables to the :root pseudo class. It matches with the root element in your document tree, generally the <html> tag. We will use :root because we want to avail the variables globally.

:root {
    --primary-color: #302AE6;
    --secondary-color: #536390;
    --font-color: #424242;
    --bg-color: #fff;
    --heading-color: #292922;

Second, we'll add our dark mode css variables.

[data-theme="dark"] {
    --primary-color: #9A97F3;
    --secondary-color: #818cab;
    --font-color: #e1e1ff;
    --bg-color: #161625;
    --heading-color: #818cab;

What is [data-theme="dark"]? This means we are referencing a data attribute called data-theme with a value "dark". We will find the use of it in a while.

Then, we can reference these variables in our stylesheets like so-

body {
    background-color: var(--bg-color);
    color: var(--font-color);

    /*other styles*/

h1 {
    color: var(--secondary-color);

    /*other styles*/

a {
    color: var(--primary-color);

    /*other styles*/

Adding the HTML "toggle switch markup"

This is essentially just a checkbox, however we will add some additional markup to style like a toggle switch. I referenced the styles from this codepen.

<div class="theme-switch-wrapper">
    <label class="theme-switch" for="checkbox">
        <input type="checkbox" id="checkbox" />
        <div class="slider round"></div>
  <em>Enable Dark Mode!</em>
/*Simple css to style it like a toggle switch*/
.theme-switch-wrapper {
  display: flex;
  align-items: center;

  em {
    margin-left: 10px;
    font-size: 1rem;
.theme-switch {
  display: inline-block;
  height: 34px;
  position: relative;
  width: 60px;

.theme-switch input {

.slider {
  background-color: #ccc;
  bottom: 0;
  cursor: pointer;
  left: 0;
  position: absolute;
  right: 0;
  top: 0;
  transition: .4s;

.slider:before {
  background-color: #fff;
  bottom: 4px;
  content: "";
  height: 26px;
  left: 4px;
  position: absolute;
  transition: .4s;
  width: 26px;

input:checked + .slider {
  background-color: #66bb6a;

input:checked + .slider:before {
  transform: translateX(26px);

.slider.round {
  border-radius: 34px;

.slider.round:before {
  border-radius: 50%;

Adding the JavaScript

The final part is to add the bit of JavaScript to tie it all together.
We have 3 tasks in hand-

  • Add event handlers to handle accordingly the check/uncheck event of toggle-switch
  • Store the user preference for future visits
  • Check for saved user preference, if any, on load of the website

Adding the event handlers

const toggleSwitch = document.querySelector('.theme-switch input[type="checkbox"]');

function switchTheme(e) {
    if (e.target.checked) {
        document.documentElement.setAttribute('data-theme', 'dark');
    else {
        document.documentElement.setAttribute('data-theme', 'light');

toggleSwitch.addEventListener('change', switchTheme, false);

Remember, the data-theme attribute we referenced in CSS above, this is where it's getting added to our root element.

Store the user preference for future visits

We will use browser's localStorage to store the user preference.
Let's modify our switchTheme function -

function switchTheme(e) {
    if (e.target.checked) {
        document.documentElement.setAttribute('data-theme', 'dark');
        localStorage.setItem('theme', 'dark'); //add this
    else {
        document.documentElement.setAttribute('data-theme', 'light');
        localStorage.setItem('theme', 'light'); //add this

Check for saved user preference, if any, on load of the website

We will check if the theme preference is saved, if yes then we will, accordingly-
- set our data-theme attribute
- check/uncheck our toggle-switch checkbox

const currentTheme = localStorage.getItem('theme') ? localStorage.getItem('theme') : null;

if (currentTheme) {
    document.documentElement.setAttribute('data-theme', currentTheme);

    if (currentTheme === 'dark') {
        toggleSwitch.checked = true;

That's it! Check out the full demo below.

I recently added this to my website, so check that out as well, maybe!

Pro-Tip: How to decide on the color scheme?
My suggestion is to stay in the same spectrum of your primary or brand color and generate a palette out of it. Use the darkest shade from the palette as the background color and the lighter shades as font colors when in dark mode. I used this color shades generator for generating my color palette.

Even if you don't end up using the exact colors generated, it is still a good place to start. Even I ended up fine tuning the colors I ultimately used.


  1. Learn CSS Variables in 5 minutes

Posted on by:


markdown guide

Just how we do it with dev.to!

CSS variables are awesome.


Wait, what! There is dark theme for dev.to, nobody told me about that. πŸ˜‚


Dark mode is new, and it's fabulous (with a few kinks that still need to be worked out).

Oh! I see. How can I enable dark mode?

Settings ➑ Misc. On that page you'll find it under "Style Customization"


Love the dark mode on dev.to! πŸ™‚


Where is bro? Just tell me where is it? I didn't know about it...


Instead of using the following instruction you used, which forces two calls to localStorage (which is pretty slow),

const currentTheme = localStorage.getItem('theme') ? localStorage.getItem('theme') : null;

you could use an or operator.

// Value at right-hand side of || is the default
const currentTheme = localStorage.getItem('theme') || null;

or, in this case, as the default is null, even simpler,

const currentTheme = localStorage.getItem('theme');

by taking the property of localStorage.getItem to return null on unknown key to directly return the expected value/default.


Yes, this makes more sense. Thanks! πŸ™‚
I'll update the demo.


Hey just found a simple error in your CSS

.theme-switch-wrapper {
display: flex;
align-items: center;

em {
margin-left: 10px;
font-size: 1rem;

You have a nested selector here, this does not work in CSS only SCSS

so to fix it I just broke out the one selector to be on its own and not nested one!

.theme-switch-wrapper {
display: flex;
align-items: center;

.theme-switch-wrapper em {
margin-left: 10px;
font-size: 1rem;


CSS variables are fantastic for use cases like this. Here's a <dark-mode-toggle> that initially respects prefers-color-scheme and allows for manual overrides. Read more about it in this article.


Just added mine over at felixparadis.com :)

Quick note about accessibility though:
If you just display:none; your input, you're making it unaccessible through keyboard navigation.
Just hide it behind the label instead and everyone can toggle πŸŽ‰
Then add aria-label="Switch visual theme" to your input and blind users can know what's it about.


Thanks for this! I haven't tried my own light/dark mode since I tried implementing it on my first bootcamp project a couple years ago. I technically got it to work, but the code was not this DRY as I made two individual buttons that would add/remove a "dark" or "light" class on the entire body element. πŸ˜‚ Your solution is much better!


I keep trying to serve it via ng serve but instead i get the error "Property 'checked' does not exist on type 'Element" referring to "if (currentTheme) {
document.documentElement.setAttribute('data-theme', currentTheme);

if (currentTheme === 'dark') {
    toggleSwitch.checked = true;



You won't believe it, but actually you can nowadays even take the operation system setting for the dark mode and apply/get that, so your users don't even need that toggle, or at least use the version they prefer, by default.
With CSS and JS…


In my case, I would offer two toggle for users which are auto mode (system dark mode) and manual Light/Dark toggle, so users can choose whatever they prefer.


Very cool.

Have you thought about the prefers-color-scheme media feature? How could this be implemented in this script? You should check that too In my opinion. The toggle would have to automatically switch to the preferred color scheme.


this doesn't work for me. console says 'toggleSwitch is null'


add the code after the switch in tags


Add the JS inside DOMContentLoaded method


Thanks for this great post Ananya. I followed the instructions and managed to get the page up and running. But i am getting a console log Error: script5007 unable to get property 'addEventListener' for undefined or null source. Could someone please help me with a solution. Thanks.


I'm planning to experiment with something like this, but using CSS only (no javascript).


Great post. I would love to add a Dark theme to my website, but probably won’t have the time 😭. Bookmarked your article anyways πŸ€“


How we can set this for all pages?


this is great and so helpful - thanks so much! I do have one question - in the js, where does "light" come from? unless I'm missing it, that's not defined in the CSS, is it?


Hi Desi!
You're right, it's not defined anywhere.
If CSS doesn't find any specific styles related to light it will default back to the normal variables defined in :root.

Why I added it?
I added it so that we can use it while checking for the saved theme in localStorage further and add it to the root element. It will also be useful in cases where you might want to add some specific styles for light apart from the default styles.

I hope this helps πŸ™‚


Awesome, thanks so much! Explanation makes perfect sense.


Super duper thanks for article.


Thanks! It will help me a lot. I've been using it wrong (but working), now you've cleared my mind. :)


Thank you Ananya! Great article, loved the tip about how to come up with a night theme :)


Thank you for taking the time to write this article, it has been a great help! Would you follow this for all types of run time theming too, or do you think there is a better way to have multiple themes which people can switch between?


Nice, simple and works well even while using .less .


I visited your website and just loved it.
Could you help me with websites like this please.
Which language have you used for Backend? Could you make a tutorial for a simple blog website please.


Thanks Ananya, it's helpful for me


I was exactly looking for something like this. Thanks


Amazing πŸ™Œ Thank you! πŸ™


Great, thanks a lot!

I added an innerHTML to change the text as well:


Nice post, Thanks for writing 😁


A big thank! I was violently disabling dark-theme-CSS-sheet via JS for my blog, and it was creepy. This solution is way, way, way... way better!

I'm happy now.


How about images like the Logo in Header & Footer? How would you suggest making this happen?


add an image source changer in the functions or make the images a background image if you wanna change them with CSS