I guess it does depend on your idea of professional as to what looks professional.
but truth be told you could skin your PHPBB forum to look exactly lie this one,
the default theme is default theme, and the only reason that it doesn't look professional is because so many people use it just straight out of the box.
in actual fact the default theme is very professional, probably one of the more professional themes available because it's been designed and tweaked and made perfect. the only reason it's found lacking and seemingly unprofessional is the amount of unprofessional sites it's used on.