Creating design system for custom theme based on Casper

I have been working on customizing my own theme for for the past two weeks.
How can create a custom theme based on Capser or Digest using my own CSS stylesheet?

I know how to create sites with HTML/CSS and I have been editing and customizing the Digest and Casper themes to understand what is possible via Code Injection from Admin panel but I want to use my own design system so that editing the site is easier for me in the future.

I have read the official docs on using gulp but didn’t find it very helpful for what I was trying to do.