speaking from personal experience i can tell you that 95% of the time, i can take a template from raw html/css and port it to pb's bbcode and it transfers just fine as long as it's coded with some of pbs other limitations in mind (i.e. i still don't think that you can import libraries or run JS with pb's bbcode)

very occasionally i have to fix a couple margins when i port over, but usually anything that breaks was a mistake on my end in converting.