I once tried to add a forum to my website but found it very difficult to get them both looking similar, getting them the same proved almost impossible.
The way forward for myself was to go the other way around and add my website to the forum by using 'custom pages'. Have a read of this knowledge base article which explains how to add new pages: https://www.phpbb.com/kb/article/add-a- ... -to-phpbb/
if you feel confident with that, the development wiki has a very good article on template syntax so you will be able to add more funcionality to those custom pages: https://wiki.phpbb.com/Tutorial.Template_syntax
If you would like to see what is possible theres a link to my site in my signature. Its all running inside of the forum software but the forum is only one part (and not the main part) of the site.