If someone has already done it, then you can just install the theme, but if you have to do it yourself, then there's a lot of work involved.
You'd start with: Knowledge Base - How to start a new phpBB2 style
and refer to: Knowledge Base - phpBB2 Template Tutorial
and Knowledge Base - Tools needed to set up and customise phpBB
and ask any specific question in the "Styles Development and Discussion" forum