Bulletproof (bp) is a template framework, which means it is, on the one hand a template for the weblog engine Serendipity (s9y), and on the other hand is meant to be used as a framework to build new templates for s9y.
The Bulletproof developers' primary design goal was to create a template which is, well, bulletproof!! It should be able to create any of the "classic" website/blog layouts, in any popular browser, on any popular platform, without falling apart. The developers are fairly confident that we have achieved that goal, but of course nothing is 100% certain.
Bulletproof makes great use of s9y's "theme options", so users can now configure almost any desirable option using just the administration panel. There is almost no need to edit any of the template's core files. Of course we cannot cover every specific custom feature every user might desire, but we are fairly sure we have covered the most desireable features.