Customizing Bootstrap 4 css

While there is not an "official" Bootstrap 4 customizer like there was in 3.x, there are several tools that can do this. These all convert from SASS to CSS, and some provide a UI if you're not familiar with SASS.

Themestr.app Customizer

Bootply Customizer

Also see this answer for details on how to customize with CSS overrides or SASS.