How to Customize Bootstrap’s Sass Files With Grunt

Today, we’ll learn how to customize Bootstrap 4’s Sass files with Grunt

Instead of Grunt, we could just as easily use another task runner like Gulp, webpack, or npm scripts. Regardless of the build tool the process of customizing Bootstrap’s source Sass files will be the same.

Note: This tutorial assumes you have some familiarity with Bootstrap 4, Sass, and build tools.

Project Structure

For the purposes of this tutorial, I’ve created a Node project. Here’s its structure: 

The project structure

If you’ve never created a Node project before, Kezz Bracey has you covered in this extensive beginner’s tutorial:

Inside our project you’ll see the “Gruntfile.js” file, in which we list some common tasks. More specifically, I’ve registered tasks which:

  • Create a static web server
  • Compile Sass to CSS
  • Monitor for changes in the .html and .scss files, and then force a web server restart.

For these tasks, I’ve installed the latest versions (at the time of this writing) of these Grunt plugins:

I’ve also installed the latest version of Bootstrap along with its dependencies (jQuery, popper.js).

I do all that by listing them in the “package.json” file, which looks like this:

When we run the project via the Command Line, Node will read through “package.json” and install everything it finds in a “node_modules” folder.

To grab all the starter files for this project, visit this Github repo and download it to your system. As soon as you’ve done so, execute the following things:

  1. Make sure you’ve installed recent versions of Node.js (my PC runs v10.15.0) and npm (my PC runs v6.7.0) in your machine. To test your machine versions, execute node -v and npm -v.
  2. Run npm install -g grunt-cli. This command will install the latest version (v1.3.2 at the time of this writing) of Grunt’s command line interface (CLI) globally. You can learn more info about what this command does in this page. Alternatively, we could have installed Grunt’s CLI locally in the project’s folder. 
  3. Navigate to the project’s root directory.
  4. Run npm install. This command will install all the project dependencies (as mentioned, it will create the “node_modules” folder) listed in the “package.json” file. 
  5. Run Grunt with grunt. Hopefully, this task will be executed correctly and you won’t encounter any errors. If errors do occur, make sure that steps 1 and 2 have been completed successfully.
  6. Open your browser and type http://localhost:3000. You should see this landing page:
The Bootstrap project landing page

In the console, you should see something like this, indicating that Grunt is running:

Grunt is running

Customizing Bootstrap 4

Now that we’ve automated the development process, let’s customize our installation of Bootstrap 4, which is located inside the “node_modules” folder:

The installation of Bootstrap 4

Every Sass variable and map in Bootstrap 4 can be overridden or extended thanks to its !default flag.

With that in mind, let’s have a closer look at the “scss” folder of our project. Here’s what’s in it:

The contents of the scss folder of our project

Inside the “_bs.scss” partial file, we’ll put our Bootstrap customizations.

Inside the “main.scss” file, we’ll include the aforementioned partial and the related Bootstrap installation, like this:

The order we declare these two things matters. Our customizations should come before Bootstrap’s Sass files.

It’s also worth noting that instead of including all of Bootstrap, a better approach might be to pick only the files needed for the project.

Changing the Breakpoints

As a first customization, let’s change Bootstrap’s default breakpoints: 

Screen Viewport Size Class Prefix
Extra Small < 576px .col-*
Small ≥ 576px .col-sm-*
Medium ≥ 768px .col-md-*
Large ≥ 992px .col-lg-*
Extra Large ≥ 1200px .col-xl-*

Their definition can be found in the “bootstrap/scss/utilities/_variables.scss” file. It looks like this:

The map related to bootstrap breakpoints

The !default flag effectively says “use the following values for this variable unless they’re already assigned elsewhere”. So to override them we just have to register our own grid-breakpoints map inside the “_bs.scss” file:

That gives us the values below:

Screen Viewport Size Class Prefix
Extra Small < 576px .col-*
Small ≥ 576px .col-sm-*
Medium ≥ 768px .col-md-*
Large ≥ 1024px .col-lg-*
Extra Large ≥ 1500px .col-xl-*

Changing Container’s Width

As a second customization, let’s change the container’s width. By default, the container (i.e. .container – not the .container-fluid) has a maximum width as shown in the table below:

Screen Viewport Size Max Container Width
Extra Small < 576px None (auto)
Small ≥ 576px 540px
Medium ≥ 768px 720px
Large ≥ 1024px 960px
Extra Large ≥ 1500px 1140px

Note: In the table above, we use the custom breakpoints declared in the previous example.

Bootstrap defines the container’s width in the “bootstrap/scss/utilities/_variables.scss” file.

The map related to the bootstrap container

Just as we did before, to override these values we register our own container-max-widths map in the “_bs.scss” file:

Note: we can only specify the container’s width in pixels. If we try to use percentages (e.g. 100%), a “Potentially invalid value” warning is thrown in the console.

With this customization in place, the following values apply:

Screen Viewport Size Max Container Width
Extra Small < 576px None (auto)
Small ≥ 576px 767px
Medium ≥ 768px 1023px
Large ≥ 1024px 1499px
Extra Large ≥ 1500px 1500px

Using Bootstrap’s Mixins

Bootstrap defines the breakpoint mixins in the “bootstrap/scss/mixins/_breakpoints.scss” file. Let’s take advantage of them and change the body background color depending on the viewport size.

Inside the “main.scss” file, after the imports, let’s put this code:

That gives us the results shown in the following table:

Screen Viewport Size Body Background Hex Color
Extra Small < 576px #17a2b8
Small ≥ 576px #28a745
Medium ≥ 768px #dc3545
Large ≥ 1024px #20c997
Extra Large ≥ 1500px #ffc107

This is slightly bizarre behavior for the real world, but you can see it in action here:

For this example we used the mobile-first breakpoint mixins, however desktop-first breakpoint mixins are also available.

Here’s the same example with the desktop-first breakpoint mixins:


In this tutorial, we covered the process of customizing Bootstrap 4’s Sass files with Grunt. We used Grunt for handling some common tasks, but in your own projects you might well prefer to use a different build tool.

I hope this tutorial has given you a solid understanding of how to theme Bootstrap 4 projects from the ground up. If you have any questions, or if there’s anything you didn’t quite understand, let me know in the comments below!

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