将论坛整合到我的网站，同时保持我的设计

I have a website which i created from scratch. I want to now add a forum to this but with the users using their excising username and password from when they registered to my site.

What options do i have of already built forum code, with integrating links and keeping my own design?

Thanks

phpBB have some plugin so you can integrate your own authentication.

phpBB also let you code your own template so you can keep the forum design consistent with your website.

this question also show the same kind of solution for vbulletin.