Creating a local Sass structure 

Before we can start compiling Bootstrap's Sass code into CSS code, we have to create some local Sass or SCSS files. First, create a new scss subdirectory in your project directory. In the scss directory, create your main project file called app.scss.

Then create a new subdirectory in the new scss directory called includes. Now you will have to copy the bootstrap.scss and _variables.scss from the Bootstrap source code in the bower_components directory to the new scss/includes directory:

cp bower_components/bootstrap/scss/bootstrap.scss scss/includes/_bootstrap.scss
cp bower_components/bootstrap/scss/_variables.scss scss/includes/

Notice that the bootstrap.scss file has been renamed as _bootstrap.scss, starting with an underscore, and has become a partial file now.

Import the files you have copied in the previous step into the  app.scss file as follows:

@import "includes/variables";
@import "includes/bootstrap";

Then open the  scss/includes/_bootstrap.scss file...