How extend some variables or behavior of Bootstrap with SASS using Elixir?

Hi,

I'm trying to extend some variables and behavior of Bootstrap(version 4.0.0-alpha although I think must be the same for the 3.3.5 version). The process I use is:

1. Use bower as follow:

   bower install jquery bootstrap#v4.0.0-alpha --save-dev
   

2. Modify the install path of the bower.json file:

   "install" :
       {
           "path": "resources/assets/vendor"
       }
   

3. Use bower-installer for create the js and css files (in Bootstrap 4.0 creates the scss file, then you need to compile).

4. Then I mayuse gulp and Elixir for compile all files. But here is the question how can I change any variable without touch the original and managed by bower files? I was using:

   elixir(function(mix){
       mix.sass(['_variables.scss', 'vendor/bootstrap.bootstrap.scss']);
   });
   

Where the _variables.scss contains only some customized vars.

How would you proceed to make changes within the original SASS variables?

In example to extend or modify the $primaryColor variable, or change the new FlexBox feature.

Thanks