The is a sample MkDocs project with a custom_dir defined to add a lightbox feature to the default mkdocs theme for displaying images in a lightbox using the Lightbox for Boostrap Plugin.

To test this example, clone it and run mkdocs serve from the root directory. Then click on the image on the homepage in your browser.

Note that this example requires at least MkDocs version 0.17. Earlier versions do not support most of the mechanisms used here. See revision 915e1ac of this guide if you're working with MkDocs 0.16.

It should also be noted that this example is using the Lightbox for Bootstrap Plugin as the mkdocs theme is built on Bootswatch, a Bootstrap wrapper. If you are not using a Bootswatch or Bootstrap theme, then you will need to use a different library for implementing lightboxes.

First, MkDocs needs to be pointed to the custom theme directory. In this example, the directory is unimaginatively named custom_theme. Therefore, the mkdocs.yml config file defines its theme like this:

theme:
  name: mkdocs
  custom_dir: custom_theme

Note that we need to explicitly define the theme which we are customizing, (via theme: mkdocs) otherwise MkDocs assumes that the theme_dir is replacing the entire theme.

The CSS and JavaScript files have been copied from version 4.0.2 of Lightbox for Bootstrap. It's important to use version 4.0.2 as more recent versions require Bootstrap 4 (MkDocs uses Bootstrap version 3).

custom_theme/
    css/
        ekko-lightbox.min.css
    js/
        ekko-ligthbox.min.js

While those files technically do not need to go in the css/ and js/ subdirectories, that is how files are organized in the parent mkdocs theme, so we've done the same to keep things neat and tidy.

Finally, to tell MkDocs to use our media files we have the file custom_theme/main.html. This file will replace/override the file of the same name in the parent theme. Therefore, at the very least it needs to inherit from the base template:

{% extends "base.html" %}

Then, via template inheritance, we define blocks to override the default blocks defined in the parent theme. For example, we need the CSS file added above to be listed in the styles block along with the other CSS files. However, we don't want to remove the other stylesheet links defined in that block. Nevertheless, the default behavior is for a block to replace the parent block of the same name. Therefore we have two options:

1. Redefine every link in the parent styles block.
2. Use a super block to have the parent styles included in our new block.

Obviously, the first option would require us to edit our custom block every time the parent theme was updated. Therefore, we've used super blocks. In this case, we want our new CSS styles to be defined after the existing styles, so we've made the super() call first:

{% block styles %}
    {{ super() }}
    <link rel="stylesheet" href="{{ base_url }}/css/ekko-lightbox.min.css">
{% endblock styles %}

And to ensure the URL is accurate for any page nested at any level in our site, we have used the base_url template variable to ensure the stylesheet is always properly linked to.

The JavaScript library we added above is included in the lib block in the same manner.

{% block libs %}
    {{ super() }}
    <script src="{{ base_url }}/js/ekko-lightbox.min.js"></script>
{% endblock libs %}

Finally, we need the JavaScript which runs on page load and defines a click event for each image. This would go in the scripts block. Rather than defining another JavaScript file and requiring the client to make another request (perhaps using the extra_javascript config setting), we can just define it as an inline script in the template.

{% block scripts %}
    {{ super() }}
    <script>
        $(document).on('click', '[data-toggle="lightbox"]', function(event) {
            event.preventDefault();
            $(this).ekkoLightbox();
        });
    </script>
{% endblock scripts %}

Again, we've made sure to use super to avoid disabling any other scripts which may be defined through the extra_javascript config setting.

The final piece of the puzzle is to have some image links which conform to the Lightbox for Bootstrap format. As Markdown simply passes raw HTML through unaltered, we've simple added an image link as raw HTML in the docs/index.html file:

<a href="https://unsplash.it/1200/768.jpg?image=250" data-toggle="lightbox" data-title="A random title" data-footer="A custom footer text">
    <img src="https://unsplash.it/300.jpg?image=250" class="img-fluid">
</a>

Perhaps a Markdown Extension could be created which could then be defined in the config file (via the markdown_extensions config option) and some simple Markdown image links would work as well. However, that is beyond the scope of this example.

GOTCHA The lightbox plugin allows navigating through an image gallery by left/right arrow keys. As this feature is turned on by default (see plugin_options) it's in conflict with MkDocs's own arrow key navigation. The result is that if you hit the left or right arrow key when the gallery lightbox is shown you will effectively navigate to the next MkDocs page!