Just out of curiosity, I tried this:
How to customize phpbb forum?
The first hit is this: Customise phpBB
That seems like it would be the best place to start.
How ever :
I am looking for any tutorial video on how to customize phpbb forum?
If you want videos, I saw some youtube ones in the results, but also at the top of the result page:
Where it says Videos:
There are so many , one could not list them all here. I don't know which ones would be best , I suppose you would just need to browse through them, and watch a few to decide which one you like.