you have conflicts between the wp css and the phpbb css. the wp css is over riding the phpbb css in places.
you have also replaced some of the phpbb html with wp html which you really shouldn't be doing.
the best thing to do for this is to start over. only use the exact code for the header and put it in place of the phpbb headerbar div.
only add the relevant css for your wp header into one or more of the phpbb css files or create a new one and call it from the head section instead of calling all of the wp css into play.
I'm baaaaaccckkkk. still doing work on donation basis. PM your needs.
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