You are going to need to find a phpbb template that is "sort of" close to
the existing layout of your site. And then you'll have to customize the
heck out of it. You'll be modifying the CSS, the HTML, the layouts of the
graphics, the graphics themselves, etc.
Google: free phpbb templates
Now the phpbb URL loads, but it says "unavailable"...
probably because you have it stuck in maintenance mode?
Anyhow, you will have to edit the templates themselves.