How to extend/modify (customize) Bootstrap with SASS

Question

I want to create a website theme based on Bootstrap. I want to extend Bootstrap's default components and change some of them. For this, I would need access to SASS variables that Bootstrap defines so I can override them.

I though of cloning Bootstrap from GitHub and changing it directly, but I heard that's actually not good practice. Could you give me some advice please?

Answer 1

Update 2022 (Bootstrap 5)

Generally speaking, customizing Bootstrap 5 SASS works the same way as it did in Bootstrap 4. However, some of the maps have changed (and new ones have been added) so you should follow the Bootstrap SASS documentation for details on how to add, remove or modify any of the maps.

Recent comments on this post indicate there's still some confusion about the order of customizations and imports. However, this concept hasn't changed...

"Variable overrides must come after our functions are imported, but before the rest of the imports"

Because of the way SASS variable defaults work, bootstrap ("the rest of the imports") should be imported AFTER any variable customizations/changes. Since your variable changes will not contain the !default flag, your changes will not be overridden by the Bootstrap defaults when bootstrap is imported at the end.

The proof is in the pudding

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Here's how to override / customize Bootstrap 4 with SASS...

Overrides and customizations should be kept in a separate custom.scss file that is separate from the Bootstrap SASS source files. This way any changes you make don't impact the Bootstrap source, which makes changes or upgrading Bootstrap later much easier.

1- Consider Bootstrap's SASS folder structure, alongside your custom.scss...

|-- \bootstrap |   |-- \scss |   |   |-- \mixins |   |   |-- \utilities |   |   |-- bootstrap.scss |   |   |-- variables.scss |   |   |-- functions.scss |   |   |-- ...more bootstrap scss files |   custom.scss 

2- In your custom.scss, import the Bootstrap files that are needed for the overrides. (Usually, this is just variables.scss. In some cases, with more complex cutomizations, you may also need the functions, mixins, and other Bootstrap files.). Make the changes, then @import "bootstrap". It's important to import Bootstrap after the changes...

/* custom.scss */      /* import the necessary Bootstrap files */ @import "bootstrap/functions"; @import "bootstrap/variables";  /* make changes to the !default Bootstrap variables */     $body-color: green;          /* finally, import Bootstrap to set the changes! */ @import "bootstrap"; 

2a (optional) - Also, you can extend existing Bootstrap classes after the @import "bootstrap"; to create new custom classes. For example, here is a new .row-dark class that extends (inherits from) the Bootstrap .row class and then add a background-color.

 /* create new custom classes from existing classes */  .row-dark {     @extend .row;     background-color: #333333;     color: #ffffff;  }  

3- Compile the SASS (node-sass, gulp-sass, webpack/NPM, etc..). The CSS output will contain the overrides! Don't forget to check the includePaths if your @imports fail. For a full list of variables you can override, see the variables.scss file. There are also these global variables.

Bootstrap SASS Demo on Codeply

In summary, here's how it works:

1_ First, when the custom.scss file is processed using SASS, the !default values are defined in the bootstrap/variables.scss. 

2_ Next, our custom values are set, which will override any of the variables that had !default values set in bootstrap/variables.scss. 

3_ Finally, Bootstrap is imported (@import "bootstrap") which enables the SASS processor (A.K.A. compiler) to generate all the appropriate CSS using both the Bootstrap defaults and the custom overrides.

For those that don't know SASS, try this tool that I made.

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Also see:
How to get 15 columns in Bootstrap 4 in SASS CSS?
Bootstrap v4 grid sizes / Sass List
Customizing Bootstrap CSS template
Extending Bootstrap 4 and SASS