This is simple starter project to help you get started quickly when making a custom Bootstrap theme.
There is a full tutorial available at hackerthemes.com: How to build your own Bootstrap themes.
This readme is the short version of that.
This repository includes a project structure with a build script that builds a custom CSS version of Bootstrap 4, using Gulp. You can clone this repository, run the Gulp task and go right into modifying variables and adding styles. There's also an HTML file that contains a neatly organized collection of Bootstrap components that fit a 1920x1080 display, so you can instantly see the outcome.
There is a short demonstration of this project on YouTube: https://www.youtube.com/watch?v=Gi5ffoE3mPc
npm install gulp-cli -g
index.html(ideally with a local development webserver)
scss/_custom-styles.scss. You can use Bootstrap's mixins here.