This is the main stylesheet for the Theme it is enqueued with the handle devdmbootstrap4-css inside the functions.php file. This is a compiled version of the Bootstrap 4 stylesheet with devdmbootstrap4 styles added into it via some SCSS files.
Added SCSS Files
Important Note: These SCSS files are added into devdmbootstrap4.scss which also includes the bootstrap.scss file. They are all compiled together into devdmbootstrap4.css