Hi Paul. I'm afraid you understood my question too literally. SPPB 3 also does not have bootstrap.min.css on board. However, it is built on the bootstrap. This bootstrap is customized and is in the core SPPB CSS file. For isolation used their own classes.
For example sppb-row = row (from stasndart Bootstrap), sppb-container = container (from stasndart Bootstrap) sppb-col = col and ect.
Good news for me would be if it were not built on the bootstrap grid system, but on CSS Grid. That would make SP PB much faster, flexible and promising. SP PB has a good potential for transferring it to CSS Grid. This potential is much better than Nicepage, the developers of which went wrong by despite the use of CSS Grid.