Bootstrap 4 source SASS files were copied to the bootstrap_theme folder in your project.

The main file for your Bootstrap theme - custom.scss - was also created there.

These SASS files are compiled into the existing Bootstrap CSS stylesheet that is linked to the page.

To customize the theme:

Open _variables.scss through the Stylesheets menu in the Style panel.
Right-click on the variable that you want to change and select Customize in custom.scss.
A copy of the variable will be added to custom.scss. Change the value of the variable there.

Important: Never change variables directly in _variables.scss. This file is a part of Bootstrap and will be overwritten when you update Bootstrap.

Check out the documentation to learn more about customizing Bootstrap with Pinegrow.