Bootswatch Theme Selector

Sun 08/25/13   21:54  in  technical

Despite my passion for light-on-dark themes, not everyone finds them as readable and enjoyable as I do. To address these concerns I investigated what it would take to add a drop-down to this website to enable dynamic selection of themes by the user.

bootswatch theme dropdown

Bootswatch Theme Dropdown

The relevant code is given, but for those interested in a complete example take a look at the website github [1].

Adding the Dropdown

The drop-down is based on a StackOverflow post [2], modified to select a Bootswatch theme dynamically.

First, the HTML component of the drop-down:

<li class="dropdown" id="theme-dropdown">
  <a href="#" class="dropdown-toggle" data-toggle="dropdown"><i class="icon-cogs icon-large"></i> Theme<b class="caret"></b></a>
  <ul class="dropdown-menu">
    <li><a href="#" class="change-style-menu-item" rel="cyborg"><i class="icon-fixed-width icon-pencil"></i> Cyborg (Default)</a></li>
    <li><a href="#" class="change-style-menu-item" rel="cerulean"><i class="icon-fixed-width icon-pencil"></i> Cerulean</a></li>
    <li><a href="#" class="change-style-menu-item" rel="cosmo"><i class="icon-fixed-width icon-pencil"></i> Cosmo</a></li>
    <li><a href="#" class="change-style-menu-item" rel="flatly"><i class="icon-fixed-width icon-pencil"></i> Flatly</a></li>
    <li><a href="#" class="change-style-menu-item" rel="journal"><i class="icon-fixed-width icon-pencil"></i> Journal</a></li>
    <li><a href="#" class="change-style-menu-item" rel="readable"><i class="icon-fixed-width icon-pencil"></i> Readable</a></li>
    <li><a href="#" class="change-style-menu-item" rel="slate"><i class="icon-fixed-width icon-pencil"></i> Slate</a></li>
    <li><a href="#" class="change-style-menu-item" rel="spacelab"><i class="icon-fixed-width icon-pencil"></i> Spacelab</a></li>

Which as-written belongs in the navbar of your Bootstrap-based site. Here I have drop-down items for my preferred Bootswatch themes, but the format should straightforward to add/remove as you see fit.

The important tidbits are the class="change-style-menu-item" and rel="spacelab" fields of the theme links, the rest is Bootstrap-specific code for putting it all into a drop-down.

Additionally, we’re going to modify the primary bootswatch theme link by adding a title to it for easy lookup from javascript later:

<link href="//" rel="stylesheet" title="main">


Now to perform the desired theme change, add the following jquery function:

/* When a theme-change item is selected, update theme */
jQuery(function($) {
    $('body').on('click', '.change-style-menu-item', function() {
      var theme_name = $(this).attr('rel');
      var theme = "//" + theme_name + "/bootstrap.min.css";

What does this do? This adds a function to each of the change-style-menu-item links we added in our HTML earlier which is triggered when the link is clicked.

When invoked, this function extracts the value of the rel attribute of the clicked link, and uses it to invoke set_theme with an appropriate replacement CSS URL.

Below is a tentative definition for set_theme that we’ll be replacing in the following section.

function set_theme(theme) {
  $('link[title="main"]').attr('href', theme);

Making The Selection Persist

What we’ve done so far adds the drop-down and lets users change the theme. That’s nifty and highlights the magic of Bootswatch themes, but what if we wanted to make the user’s selection persist across visits and as they navigate the site?

To accomplish this I opted to use HTML5’s local storage [3] feature. Being a simple blog without a concept of users, stashing this server-side makes little sense, and local storage is supported by all modern browsers and is very easy to use.

Saving Theme with Local Storage

First, we add a function to determine if the user supports the local storage feature. This helps avoid errors on browsers without support or with the feature disabled:

function supports_html5_storage() {
  try {
    return 'localStorage' in window && window['localStorage'] !== null;
  } catch (e) {
    return false;

var supports_storage = supports_html5_storage();

Next let’s replace our set_theme() function with one that saves the selected them into local storage:

function set_theme(theme) {
  $('link[title="main"]').attr('href', theme);
  if (supports_storage) {
    localStorage.theme = theme;

Finally, add code to load the setting and apply it if we find the user has a saved theme choice:

/* On load, set theme from local storage */
if (supports_storage) {
  var theme = localStorage.theme;
  if (theme) {
} else {
  /* Don't annoy user with options that don't persist */

Where I chose to hide the drop-down altogether for clients that don’t support local storage.

Selectively Enabling on Development Builds

Ultimately I decided to not publish this on the production version of the website, only enabling it in development builds. A website’s design is an important part of capturing the author’s voice and by giving control over this to visitors the expressivity of the blog is weakened. Kudos to my friend Brian for pointing this out.

In this section I describe the easy Pelican-specific changes needed to only include the theme-selection code in development but not in production builds.

First, I added the following new definition to


and the following to


Next, I wrapped the various HTML and javascript components in base.html with

{% endif %}

Which has an effect very much like C preprocessor #ifdef THEME_CHANGER#endif, only including the theme switcher dropdown and supporting javascript when not using the publish configuration.

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