Toggle light and dark themes in Bootstrap

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In this article we will learn how we can easily toggle between light and dark theme.

Themes in Bootstrap

While theming in Bootstrap can be done through it's built-in SASS variables, for this article we are going to use provided css stylesheets.

Bootswatch has some great themes, we're going to use it's Cyborg theme for darker variant. And for light, we are going to use Bootstrap's default theme.

So, let's get started.

Create project folder and index.html file

mkdir toggle-bootstrap-theme
cd toggle-bootstrap-theme

Create the index.html file in it:

<!DOCTYPE html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8" />
    <meta
      name="viewport"
      content="width=device-width, initial-scale=1, shrink-to-fit=no"
    />

    <!-- Bootstrap CSS -->
    <link
      rel="stylesheet"
      href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css"
      integrity="sha384-JcKb8q3iqJ61gNV9KGb8thSsNjpSL0n8PARn9HuZOnIxN0hoP+VmmDGMN5t9UJ0Z"
      crossorigin="anonymous"
    />

    <!-- 🚨 Notice this link -->
    <link id="dark-theme-style" rel="stylesheet" />

    <title>Toggle Bootstrap Theme</title>
  </head>
  <body>
    <nav class="navbar navbar-transparent">
      <!-- 🚨 Notice the toggleTheme() function -->
      <a
        href="javascript:void()"
        class="btn btn-outline-info btn-lg ml-auto font-weight-bold"
        id="theme-toggler"
        onclick="toggleTheme()"
      ></a>
    </nav>

    <div class="container-fluid">
      <div class="jumbotron">
        <h1 class="display-4">Hello, world!</h1>
        <p class="lead">
          This is a simple hero unit, a simple jumbotron-style component for
          calling extra attention to featured content or information.
        </p>
        <hr class="my-4" />
        <p>
          It uses utility classes for typography and spacing to space content
          out within the larger container.
        </p>
        <a class="btn btn-primary btn-lg" href="#" role="button">Learn more</a>
      </div>
    </div>

    <!-- Optional JavaScript -->
    <!-- jQuery first, then Popper.js, then Bootstrap JS -->
    <script
      src="https://code.jquery.com/jquery-3.5.1.slim.min.js"
      integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj"
      crossorigin="anonymous"
    ></script>
    <script
      src="https://cdn.jsdelivr.net/npm/popper.js@1.16.1/dist/umd/popper.min.js"
      integrity="sha384-9/reFTGAW83EW2RDu2S0VKaIzap3H66lZH81PoYlFhbGU+6BZp6G7niu735Sk7lN"
      crossorigin="anonymous"
    ></script>
    <script
      src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js"
      integrity="sha384-B4gt1jrGC7Jh4AgTPSdUtOBvfO8shuf57BaghqFfPlYxofvL8/KUEfYiJOMMV+rV"
      crossorigin="anonymous"
    ></script>

    <!-- 🚨 Our custom JavaScript  -->
    <script src="index.js"></script>
  </body>
</html>

Notice these 2 snippet in above code:

<link> to dynamically load stylesheet

<link id="dark-theme-style" rel="stylesheet" />

Here, we will render dark theme stylesheet through JavaScript. Also note that we have kept this after our default stylesheet, so that if any rules is missing in dark one, it will be taken from default.

<a> to toggle themes

<a
        href="javascript:void()"
        class="btn btn-outline-info btn-lg ml-auto font-weight-bold"
        id="theme-toggler"
        onclick="toggleTheme()"
></a>

This anchor tag will help user to toggle between light and dark theme. We are going to create function toggleDark in JavaScript. Let's see that.

Create index.js file

// you can use app's unique identifier here
const LOCAL_STORAGE_KEY = "toggle-bootstrap-theme";

const LOCAL_META_DATA = JSON.parse(localStorage.getItem(LOCAL_STORAGE_KEY));

// you can change this url as needed
const DARK_THEME_PATH = "https://bootswatch.com/4/cyborg/bootstrap.min.css";

const DARK_STYLE_LINK = document.getElementById("dark-theme-style");
const THEME_TOGGLER = document.getElementById("theme-toggler");

let isDark = LOCAL_META_DATA && LOCAL_META_DATA.isDark;

// check if user has already selected dark theme earlier
if (isDark) {
  enableDarkTheme();
} else {
  disableDarkTheme();
}


/**
 * Apart from toggling themes, this will also store user's theme preference in local storage.
 * So when user visits next time, we can load the same theme.
 *
 */
function toggleTheme() {
  isDark = !isDark;
  if (isDark) {
    enableDarkTheme();
  } else {
    disableDarkTheme();
  }
  const META = { isDark };
  localStorage.setItem(LOCAL_STORAGE_KEY, JSON.stringify(META));
}

function enableDarkTheme() {
  DARK_STYLE_LINK.setAttribute("href", DARK_THEME_PATH);
  THEME_TOGGLER.innerHTML = "🌙 Dark";
}

function disableDarkTheme() {
  DARK_STYLE_LINK.setAttribute("href", "");
  THEME_TOGGLER.innerHTML = "ğŸŒž Light";
}

I believe that above code is self-explanatory 😉 and no further explanation is required.

Output

After successfully writing all of the above code, you can simply open index.html in browser to see the output:

Toggle Bootstrap Theme Output

👉 The http call to dark stylesheet is done only once per session. If user switches back to light theme and then again dark theme, it is loaded from cache. See the Network panel in browser devtools to see this.

Conclusion

We learned how easily we can toggle between light and dark theme with just few lines of JavaScript code.

If you're looking for a complete Bootstrap theme generation code base with support of sass, gulp, auto-refresh, etc. checkout my github repo:

And yes, always believe in yourself...

silhouette of trees under starry night photo – Free Nature Image on Unsplash

Photo by Nick Dunlap on Unsplash

Comments (4)

Johan Fredrik Varen's photo

This is a beautiful way of doing it, except for the one major flaw: The flash of light-mode at every page (re)load. I've tried several approaches, but as long as you set the dark-mode stylesheet URL using JS you'll either have a bright flash of unstyled content or a bright flash of light-mode content. This is even visible in the GIF shown in this article. I'm afraid I've no choice but to ditch this approach and resort to backend rendering.

Show +1 replies
Johan Fredrik Varen's photo

Dharmen Shah Thanks, but I tried that. Chrome seems to always first draw a white viewport unless it has the dark mode CSS already cached. I ended up with backend rendering like I said, with a cookie to hold the state instead of local storage. Works flawlessly in practice, but it feels old-school and I wish your approach would have worked as I think it's a more appropriate SoC fulfillment.

Dharmen Shah's photo

Johan Fredrik Varen Ok. I tried setting up variables through JS in my head section, it seems to be working fine.

I took that approach on: gitalytics.shhdharmen.me.

Overall, it goes like this:

<html>
<head>
...
<script type="text/javascript">
      let root = document.documentElement;
      const isDark = localStorage.getItem('gitalytics-isDark');
      if (isDark === 'true') {
        root.style.setProperty('--bg-body', '#303030');
        root.style.setProperty('--color-body', 'white');
      } else {
        root.style.setProperty('--bg-body', '#fafafa');
        root.style.setProperty('--color-body', 'rgba(0, 0, 0, 0.87)');
      }
    </script>

    <style type="text/css">
      body,
      html {
        background-color: var(--bg-body, #fff);
        color: var(--color-body, rgba(0, 0, 0, 0.87));
        height: 100%;
        transition: background-color 0.25s ease-in-out, color 0.25s ease-in-out;
      }
</style>

</head>
<body>
...
</body>
</html>