honestly the best way I know of to style/mod is to simply know the code. Or atleast be able to read and understand it. The problem with coding is everyone has there own ways on how to code something. Course thats not always a bad thing.
if you wish to learn the code.. i can get you started there...
Code PhpBB uses (that i know):
depending on which aspect you like to be able to do.
For modding you need to know all of them really depending on what the mod is.
SQL you don't really need as much as the rest.