Yeah, of course, you can use the BP styles all the same and it would
make sense if the design allows for it. That way you can minimize the
custom CSS you may have to do and keep your code consistent which will
be great for working on it in the future. However, if you do create
some custom CSS I'd recommend you comment it well and if possible keep
it consistent with the principles that BP uses - this will help you
conceptually and whoever else might touch the code won't have as hard
a time deciphering what you've done.