How To Get WordPress To Pickup Boostrap’s CSS File?

Don’t know what happened and I don’t see an answer.

Basically, I had to use another them and one-by-one, change all the files, then download it and make it my own. It references bootstrap in the functions.php the same way that I do, yet it works and my theme didn’t. Every file has been completely scrubbed and replaced with my code, so I am at a loss why that worked, since none of the theme’s original code remained.