won't get any decent answers to this question. it is completely subjective.
the one I might think is absolutely wonderful, you might think is crap.
no matter what answers you get the one that is the best for you is whichever one you like.
in reality, your users will not care about the style. ( by the way, they are not called themes in phpbb, they are called styles) They will care about the content. If you don't have a lot of content that is related to your subject matter, they won't stay long enough to even register, regardless of what it looks like.
I'm baaaaaccckkkk. still doing work on donation basis. PM your needs.
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If nobody is in the forest, does a tree really fall?