I have checked a lot of the sites that are listed in the promotion section and I have honestly wanted to tell each one how to make modifications on their phpbb forum.
Although I am not an expert I did modify my forum before so I know a bit when it comes to customization.
Customization is like building a house you take it per part.
1) Try to think what your site would look like (Visualize)
2) Make sure you have to correct tools (Photoshop and Dreamweaver. You can also download the buttons and banners if you like.)
3) Get the tutorials on how to modify it (Again do it per part. I would start with getting rid of that annoying phpbb banner.)
Reference: Click here