Hi,
If you used the Menu module (Joomla core), you can add class names from Bootstrap documenation to it, to make some basic changes or create your own class name with a more unique style.
If that module will be used on whole site (every page), you have to add Custom CSS inside Template, not by SPPB component.
Read Helix documenation > https://www.joomshaper.com/documentation/helix-framework/customization-tips
If on one Page only, then indeed, you can use Page Settings > Page CSS > Custom CSS for it. But easy to forget about that after a week. That's why using Template > Custom CSS is more recommended for the long term.