At some point, you decided to use the Bootstrap CDN starterkit theme provided by the Bootstrap Drupal Theme. You probably ran across a google pagespeed of your site, and they told you to get rid of those external loading files like bootstrap.min.css and bootstrap.min.js.
Going back to your site, you changed the CDN delivery method to "None" and reloaded your site, only to watch in the horror of expectations that the theme just blew up... So you go back and quickly turn on the CDN, refreshed and sighed a big relief.
Then you decided to see if someone else had done the same thing and maybe figured it out.
That's pretty much what happened to me, and yup, I found where someone figured it out. I'm going to re-post their steps with a few minor tweaks and better explanation, but all the kudos go to KimNyholm, the guy figured it out. If you're using a bootswatch theme, I can't help too much (yet) but you might check out that link for some tips.
This is designed to work with the "Bootstrap" theme (not tested against the bootswatch themes like "Journal")
In theory, you can grab the bootstrap.min.css from the legacy bootswatch cdn and simply overwrite the one in the following steps.
Perfect and simple
Perfect and simple explanation!