Great question! Bootstrap 3 had a customize page as you noted.
You could use this build tool: https://bootstrap.build/. I'm not sure if it is the replacement for the BS3 customize page or something new. It looks like a nice editor.
Or you could build a custom version of Bootstrap 4 with SASS.
This link provides more information about making a theme:
https://getbootstrap.com/docs/4.0/getting-started/theming/
