Have wordpress installed and checkout this repository inside wp-content/themes/ (then enable understrap theme in WP settings).
This theme is based on understrap wordpress theme which utilizes Bootstrap 4 CSS framework.
Run gulp watch while developing (this will compile sass to css on the fly).
The main css is in sass/theme/_theme.scss and variables in sass/theme/_theme_variables.scss (you can override Bootstrap 4 variables here as well, take a look at Bootstrap 4 documentation for more info).
Run gulp dist for deploy, this will compile the theme in the dist/ directory, then upload dist/ to production wordpress' themes/ directory.