I'm generally a fan of having the forum use a theme similar to the main site, personally. Back then, that was often a suggestion from me on MaxCoderz TI forums, as they used the default Ikonboard and later PhpBB2 theme, which didn't match the site. But it depends if you plan to get a very active board, tho, because in that case it might be good to have a light theme for the forums along with the dark one.