I guess it depends on the structure of the theme, but essentially you need to try and match the CSS elements in the theme’s stylesheet to their bbPress counterparts. Some of it will be redundant, and personally I found it easier to just take the bulk of the general styling and then add in the rest later. Didn’t turn out perfect though. I’d recommend using Kakumei as a base as well.
start with the header and footer, keep the “middle part” as it is.
if you can modify create themes for WP, than it’s not so hard to do the same for bbpress
This is how I’m doing it. Please, this is not the official how to on this.
PART 1
COPY Kakemi theme into my-template. Open Style.css and change this line, Theme Name: Kakumei
I changed it to Theme Name: Kakumei (Edited Theme)
Activate it, you’ll see Kakumei’s screen shot but Kakumei (Edited Theme) as it’s name.
Then open style.css of the Edited Theme and open the CSS of the WordPress theme. Their might be multiple css so look out. I make a line in the style.css at the bottom.
/*********************** WPTHEME *******************
Then start with Header.php and footer.php start copying over the codes from WP theme to BBpress theme. Slowly start looking at the classes it that the header uses and start bringing in the codes from the WP to BBpress CSS. BEWARE some of them uses the same class! so rename the WP one not the BBpress one!!! You will have to replace all the div tags and such. The only one I recommend to keep in the header is the <div id="main"> at the end. The same with the footer. but remember to put an extra </div> on the top to cover the <div id="main"> in the header. Start with that. If you can get the hang of it then you’ll be able to continue.
I’ll keep plugging along. I come from a Sys Eng background and this is my first dive into working on the viewable side of things 
Step two
copy your images folder in the WP theme folder to your BB theme folder.
Interesting thing sometimes worth knowing: you don’t have to copy everything from Kakumei, since anything not found in your theme will be used from there. I find it helps to reduce clutter if you only copy files as you need to edit them, since you can work out what might have gone wrong later.
Do I have to make everything absolutely positioned or somesuch? Because a lot of the stuff in my style.css doesn’t seem to be being applied as I intend, or at all, and I can’t figure out why. That would make it MUCH more difficult to port a WordPress theme where everything is positioned with only “clear” and margins. I’ve made another thread about this.
