I wrote a small guide to customize Bootstrap using Hugo pipes here: https://simpleit.rocks/golang/hugo/customizing-bootstrap-with-hugo-pipes/
Basically you customize the Bootstrap SASS variables, then load the whole package and finally import your custom styles.
In assets/sass/main.scss
@import "custom_variables"; //<-- where BS customization takes place
@import "../../node_modules/bootstrap/scss/bootstrap";
@import "styles.scss"
There is a repo with a basic site and the above method at https://github.com/marcanuy/hugo-pipes-bootstrap
You can see it in action here: https://marcanuy.github.io/hugo-pipes-bootstrap/
I wasn’t able to find a way to include javascript files directly from node_modules
directory, creating symlinks didn’t work also, I think it would be related to only being supported at the very first level according to this issue so I have to use a script to copy them to the assets folder before, if there is another way to include them directly please let me know.