For most of my users, this post will be irrelevant. The bb-fv class is deprecated and probably won’t work with the latest version of WordPress. Unless you deliberately added it, then there is nothing for you to do.
The bb-fv class is one of the original features of the Big Themes. It seemed like a great idea when I built it. But it has proven itself to be unpopular and difficult to maintain. WordPress has improved a lot, and the features of the bb-fv class can be reproduced with the WordPress core block features and the background video style option.
The bb-fv class was only available in the earliest versions of the Big Themes. I did try to maintain this class through several WordPress versions, but it seemed as though nobody was using it, and it’s features could be reproduced in a more sustainable manner.
If you are using an older version of WordPress, then the bb-fv class should still work. But you will likely have problems if you update. If you are one of the few people that ever tried using this feature, then you should update your videos using Method 1. You can use the WordPress default controls to auto play your videos in excerpts.
It is also worth noting that the page level background video feature was dropped completely starting with the Big Patterns theme. The layout options made it unnecessary. However, site level and homepage background videos are still available in the latest themes. You can set these features using the customizer.
Please contact me with any questions. I apologize for any inconvenience that this may cause.