Add Dark Mode to your Websites with CSS

It's super easy to include a dark theme for your existing websites using CSS. In this tutorial, we're going to do this by taking advantage of CSS Variables.

We're going to have 3 different options for the theme - Auto, Light and Dark. The Light and Dark themes are pretty self explanatory, but the Auto theme is going to use the operating system theme setting to decide whether the site is going to be Light or Dark.

While I won't be showing you how to do this particular layout or include the content, here is an example of what we could create:

Adding the HTML

Let's start with the HTML, you can think of the value attribute as being the identifier for each theme:

<select id="theme">
    <option value="auto">Auto</option>
    <option value="light">Light</option>
    <option value="dark">Dark</option>
</select>

Adding the CSS

Let's now add a bit of CSS to the body element, here is where you specify your colors for the Light Theme using CSS Variables:

body {
    --background-color: #ffffff;
    --text-color: #000000;
}

Next, you'll want to make use of your CSS Variables throughout your style sheet - this is key to how our solution is going to work. For example, you might do:

.main-content {
    background: var(--background-color);
    color: var(--text-color);
}

button {
    color: var(--text-color);
}

We're going to be implementing a dark theme by simply replacing the values of the above declared variables in instances where we're going to be using a dark theme. Let's add this CSS:

:root {
    --dark-background-color: #111111;
    --dark-text-color: #eeeeee;
}

body.theme-dark {
    --background-color: var(--dark-background-color);
    --text-color: var(dark-text-color);
}

Now, if you add the theme-dark class to your <body> element, you should see the dark theme working. Shortly we'll be using JavaScript to toggle this value with the <select>, but let's implement our Auto option now:

@media (prefers-color-scheme: dark) {
    body.theme-auto {
        --background-color: var(--dark-background-color);
        --text-color: var(--dark-text-color);
    }
}

The above CSS uses Media Queries which is checking if the operating system prefers a Dark Theme, and if so, we want to apply the nested ruleset for body.theme-auto.

We're basically saying "Does the operating system prefer dark mode, and does the <body> have a class of theme-auto? If so, let's use Dark Mode."

Try it out by changing your OS theme color, or even better, view the website on your phone with Dark Mode enabled.

Adding the JavaScript

Now that our CSS is working, we can move onto getting our theme selector drop-down to work. Let's add the following JavaScript:

function applyTheme(theme) {
    document.body.classList.remove("theme-auto", "theme-light", "theme-dark");
    document.body.classList.add(`theme-${theme}`);
}

document.addEventListener("DOMContentLoaded", () => {
   document.querySelector("#theme").addEventListener("change", function() {
        applyTheme(this.value);
   });
});

Here, we are waiting for the DOM to be ready for us to start using it, and once it's ready, we are listening for when the user chooses an option in the theme selector drop-down. Once they choose an option, we remove all existing theme classes from the <body> (if any) and then simply add the selected theme with this.value.

A step further - remembering the theme

We could take this a step further and have the ability for the browser to remember the theme that was chosen upon a page refresh. To do this, we can use Local Storage

Let's add the following JavaScript, so we end up with this:

document.addEventListener("DOMContentLoaded", () => {
    const savedTheme = localStorage.getItem("theme") || "auto";

    applyTheme(savedTheme);

    for (const optionElement of document.querySelectorAll("#theme option")) {
        optionElement.selected = savedTheme === optionElement.value;
    }

    document.querySelector("#theme").addEventListener("change", function () {
        localStorage.setItem("theme", this.value);
        applyTheme(this.value);
    });
});

Now, upon choosing a theme, we save the theme to Local Storage by using localStorage.setItem("theme", this.value). Following this up, on page load, we grab the previously saved theme into the savedTheme constant, with a default of auto. Once we have this, we simply apply the saved theme.

Adding to this, we are then looping through each one of our <option> elements and checking to see if the value is that of our saved theme, and if so, choose that option as "selected".

To test if it works, refresh the page, choose a theme, refresh again, and your theme should stick!