_custom.scss alternatives

Created by: silverwind

Now that _custom.scss has been remove in https://github.com/twbs/bootstrap/pull/22821, I wonder how builds with custom colors and the likes could be made. My build so far was

Clone twbs/bootstrap to a temporary location
Copy my _custom.scss into the scss directory
Run npm build scripts
Copy the compiled .css and .js files to my project

The PR mentions it was nuked "in favor of relying on imports from package manager directories" but I have a hard time following? Anyone care to explain?