Adds the Twitter Bootstrap framework to your Jekyll site.
Although the Twitter Bootstrap - framework exists as a GEM, called Bootstrap Ruby Gem, it cannot be used directly in a Jekyll site, because it is specifically made for the Rails asset pipeline.
bootstrap4jekyll integrates the Bootstrap Ruby Gem into Jekyll. Thus making the Bootstrap Ruby Gem available to Jekyll users.
This GEM is inspired by the jekyll-bootstrap-sass GEM, which has not been maintained for a while.
Add the following to your site's Gemfile:
group :jekyll_plugins do
  gem "bootstrap4jekyll", "~> 1.0"
end
Now run Bundler:
$ bundle install
This will add the bootstrap4jekyll plugin to your site. Together with the plugin also the
Twitter Bootstrap - framework 
will added to your site.
When you plan to build a Jekyll theme, instead of declaring the plugin in the Gemfile you 
should declare it in your *.gemspec file. Like this:
  spec.add_runtime_dependency "bootstrap4jekyll", "~> 1.0"
And additionally in your _config.yml file, like this:
plugins:
  - bootstrap4jekyll
When you use this theme in a Jekyll site the bootstrap4jekyll plugin 
and the Twitter Bootstrap - framework
will be automatically loaded 
together with your theme.
To customize the Twitter Bootstrap - framework,
you can proceed as explained in the Jekyll Docs
about Sass assets.
Create a .scss file (e.g., assets/myStyle.scss), with the following:
---
---
@import 'bootstrap';
// (Your custom SCSS Here)
When your site is built, Jekyll will automatically add the Twitter Bootstrap - framework 
before it renders 
your site's css. In the above example, the resulting file would be assets/myStyle.css 
with Bootstrap's CSS followed by your own.
By default, bootstrap4jekyll will simply make the @import 'bootstrap'; directive available 
to your custom stylesheets so that you can more easily include Bootstrap's CSS.
If you would like to use Bootstrap's javascripts, you'll need to add the 
following to your site's _config.yml:
bootstrap:
  assets: true
This will create assets/javascripts/bootstrap folders in the generated site, 
which you can include in your site's header as you would any other javascript file.
bootstrap4jekyll relies on official Bootstrap Ruby Gem.
By default, the newest version starting from 4.2 will be used.
To use a specific version of the Bootstrap Ruby Gem, simply specify the desired version in your Gemfile.
Like this:
  gem 'bootstrap', '~> 4.5'
To see how the version of the Bootstrap Ruby Gem relates to the version of the included framework, look into the GEM's changelog