If you're converting a style from another bulletin board software, you first need the permission from the original style author. If you go ahead and do it anyway, if they find out you're likely to end up with some form of copyright infringement complaint coming your way.
If you get the permission, the main files you need to edit are: overall_header.html, overall_footer.html, colours.css and perhaps common.css, index_body.html and forumlist_body.html . There's a 90% chance you won't need to edit anything other than those.
Don't expect us to tell you how to do it step-by-step though, we can offer pointers but we wont do it for you