Ananya Neogi

Posted on Apr 10, 2019

Create A Dark/Light Mode Switch with CSS Variables

#webdev #css #showdev

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>
  </label>
  <em>Enable Dark Mode!</em>
</div>



/*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 {
  display:none;
}

.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.

Resources

Learn CSS Variables in 5 minutes

Top comments (61)

Ben Halpern • Apr 10 '19

Just how we do it with dev.to!

CSS variables are awesome.

Yashu Mittal • Apr 11 '19

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

Ben Halpern • Apr 11 '19

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

Yashu Mittal • Apr 12 '19

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

Ananya Neogi • Apr 12 '19

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

Yashu Mittal • Apr 14 '19

Awesome, thanks. Looks cool.

Ananya Neogi • Apr 11 '19

Love the dark mode on dev.to! 🙂

gevv • Oct 15 '20

@Ananya Neogi

Hi,

Dark mode does not not work link open new window (target="_blank" links)

is there a solution?

Thanks
Best regards

Zonayed Ahmed • Jun 17 '19

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

MrCaspan • Feb 10 '20 • Edited

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;
}

Thomas Steiner • Aug 9 '19

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.

Félix Paradis • Jun 7 '20 • Edited

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 🎉
Then add aria-label="Switch visual theme" to your input and blind users can know what's it about.

Ananya Neogi • Apr 11 '19

Yes, this makes more sense. Thanks! 🙂
I'll update the demo.

Liz Jewell • Apr 13 '19

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. 😂 Your solution is much better!

Bhupesh Varshney 👾 • Apr 11 '19

Nice post 👌👌 🤩
Gonna embed into my portfolio

Kiberu-sys • Jul 23 '20

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.

Simon Coudeville • May 7 '19

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.