Guide to add Bootstrap 4 to Jekyll with focus on having also a CSS stylesheet using its variables and custom ones.


The real power of Bootstrap is to use and redefine its variables in our custom designs, that means, we should not simply add Bootstrap’s javascript and CSS stylesheets to use its components, we need to change them and not making all the web look boringly the same.

This is a guide to make it easy to use Bootstrap 4 with a Jekyll website and make it easy to customize it using its variables and defining new ones.


Jekyll provides built-in support for syntactically awesome stylesheets (Sass).

Sass is a CSS extension language, it provides:

  • Variables
  • Nesting elements
    • Loops
    • Arguments
  • Selector inheritance

It consists of two syntaxes:

  • the original, indented, syntax uses the .sass extension.
  • the newer syntax, more similar to CSS, uses .scss extension.

To make Jekyll process these SASS files, we need to create files with the proper extension name (.scss or .sass) and start the file contents with two lines of triple dashes.

A file named assets/main.scss will be rendered like assets/main.css.

As Bootstrap switched from Less to Sass1 now we can use it directly without relying in parallel projects like bootstrap-sass used with Bootstrap 3.

Installing Bootstrap 4

We will be using the package manager Bower to install Bootstrap. At our Jekyll website root folder we use the bower install command. In this case I will be using bootstrap#v4.0.0-alpha.6 but you can find the latest one at

$ bower install bootstrap#v4.0.0-alpha.6
bower bootstrap#v4.0.0-alpha.6  cached
bower bootstrap#v4.0.0-alpha.6         validate 4.0.0-alpha.6 against
bower jquery#>=1.9.1                     cached
bower jquery#>=1.9.1                   validate 3.1.1 against>=1.9.1
bower tether#^1.4.0                      cached
bower tether#^1.4.0                    validate 1.4.0 against^1.4.0
bower bootstrap#v4.0.0-alpha.6          install bootstrap#4.0.0-alpha.6
bower jquery#>=1.9.1                    install jquery#3.1.1
bower tether#^1.4.0                     install tether#1.4.0

bootstrap#4.0.0-alpha.6 bower_components/bootstrap
├── jquery#3.1.1
└── tether#1.4.0

jquery#3.1.1 bower_components/jquery

tether#1.4.0 bower_components/tether

Adding new Sass load paths

We need to add this new path so Jekyll can process its Sass files.

Jekyll will look at the folder specified by the sass_dir configuration key (/_sass by default), but it also supports extending it, so it will process other folders too.

Instead of setting a custom sass folder with:

    sass_dir: _sass

we use the load-paths2 key in _config.yml to add more paths:

        - _sass
        - bower_components

load_paths only works when not in safe mode3

Add javascript

Add Bootstrap core JavaScript, JQuery and Tether4 (already installed when installing Bootstrap) at the end of the document so the pages load faster, just before the </body> HTML tag.

We add them in the default layout at _layouts/default.html or in footer.html in the _includes folder:

<script src="{{'/bower_components/jquery/dist/jquery.min.js' | prepend: site.baseurl}}"></script>
<script src="{{'/bower_components/tether/dist/js/tether.min.js' | prepend: site.baseurl}}"></script>
<script src="{{'/bower_components/bootstrap/dist/js/bootstrap.min.js' | prepend: site.baseurl}}"></script>

Import Bootstrap and use Sass variables

Create variables Sass partial

To be able to define new variables and reuse the ones defined in Bootstrap, we create a new partial Sass file _sass/_variables.scss.

  1. We define our variables
  2. “Overwrite” the ones we want from Bootstrap bower_components/bootstrap/scss/_variables.scss before loading them and then
  3. we import the Bootstrap variables.

All the variables defined in Bootstrap 4 have the !default5 property at the end. When Jekyll process each Scss file, it only defines the variables that do not have been assigned any value yet, so we can define Bootstrap’s variables before Bootstrap itself define them. It is important to do this before importing the variables because there are many of them depending on each other to calculate CSS properties values.

In _sass/_variables.scss:

$custom-font-size: 20px;
@import "../bower_components/bootstrap/scss/variables";

Import variables from main Sass file

After we have our variables, we import them from our main style sheet:

In assets/main.scss we import them and then work with our styles, using the above variables:


@import "variables";
@import "bootstrap/scss/bootstrap";

.content {
  font-size: $custom-font-size;

Don’t miss the triple dashes at the beginning of the file to ensure Jekyll reads the file to be transformed into CSS later

Add css to layout

After we have our assets/main.scss, Jekyll will process it and generate the final CSS file: assets/main.css.

That is the path we need to add to our layout, in the <head> section of _layouts/default.html we include the css: <link rel="stylesheet" href="/assets/main.css">

<!-- site css -->
<link rel="stylesheet" href="/assets/main.css">

All together

This is the basic flow Jekyll follows processing these Scss files to generate assets/main.css:

graph TB STYLESSCSS["Jekyll reads /assets/main.scss"]-->partials{"imports Sass partials"} partials--> |"import variables"|VARS partials--> |"import bootstrap/scss/bootstrap"|BS_SCSS VARS["/_sass/_variables.scss"]-->BS_VARIABLES BS_VARIABLES["@import '../bower_components/bootstrap/scss/variables';"]-->STYLESCSS BS_SCSS["/bower_components/bootstrap/scss/bootstrap.scss"]-->STYLESCSS STYLESCSS["Generates /assets/main.css"]


You’ll find a minimal example of a site in Jekyll hosted on GitHub, based on this article and ready to use, and the following step by step video:

It is also part of a Jekyll starter site jekyll-skeleton.


When we build our site, Jekyll will process the .scss files with our custom variables and we will have them in our assets/main.css. In this example its content starts with the Bootstrap code and ends with our custom _main.scss processed, looking like:

 * Bootstrap v4.0.0-alpha.6 (
 * Copyright 2011-2017 The Bootstrap Authors
 * Copyright 2011-2017 Twitter, Inc.
 * Licensed under MIT (
/*! normalize.css v5.0.0 | MIT License | */
html {
  font-family: sans-serif;
  line-height: 1.15;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%; }


.content {
  font-size: 20px; 



    Moved from Less to Sass. Bootstrap now compiles faster than ever thanks to Libsass, and we join an increasingly large community of Sass developers.

  2. Issue referring the code at 

  3. Safe mode disables custom plugins, and ignores symbolic links.

    The reason multiple load paths are shut off in safe mode is because of a fundamental inability to trust non-user content.

  4. Tether is a small, focused JavaScript library for defining and managing the position of user interface (UI) elements in relation to one another on a web page used by Bootstrap 

  5. SASS Variable Defaults: !default

    You can assign to variables if they aren’t already assigned by adding the !default flag to the end of the value. This means that if the variable has already been assigned to, it won’t be re-assigned, but if it doesn’t have a value yet, it will be given one.

    For example:

    $content: "First content";
    $content: "Second content?" !default;
    $new_content: "First time reference" !default;
    #main {
      content: $content;
      new-content: $new_content;

    is compiled to:

    #main {
      content: "First content";
      new-content: "First time reference"; }

Marcelo Canina
I'm Marcelo Canina, a developer from Uruguay. I build websites and web-based applications from the ground up and share what I learn here.