Thanks for your reply - I should have explained better! I was referring to a Bootstrap Studio Template like "Landing Page". For example, in that template, there is a css class named "masthead" and it has been incorporated into the BS4 CSS file. Which means that this class is only available if I use the BSS file instead of a standard BS file from a CDN.
as you can see, "header.masthead" is "locked" into Bootstrap. I could copy these classes one-by-one and put them into a styles.css file, but that seems pretty awkward. So, I was wondering if some of these templates used a styles file instead of creating a customized BSS file?