SO you just want the themes to match, not get a feed of your phpBB posts into WordPress or vice versa?
This isn’t too hard to do, but it is time consuming, phpBB themes are rather complicated compared to most WordPress themes.
Most of what you will have to edit is the CSS stylesheet of the phpBB theme. Depending on how different the themes, you might have to make some changes to the phpBB theme’s template files.
For some parts you could take the HTML that WordPress generates from your theme to make the pages it servers to the browser and add it to your forum theme. For example if you had a footer on your blog theme that you wanted to use on your forum theme. You could just view the source in your browser, snag the footer code and add it to your phpBB theme’s footer template page. Firebug is handy for figuring out which bits to cut and paste!
But you won’t be adding any of the tags that WordPress uses or anything, those won’t work without WordPress. And all of the elements in phpBB will have different names than in WordPress. So you will have to go through and totally redo the forum theme with the colors and other values from your WordPress theme.
