PHPBB themes, are basically html.
there located in templates/
SubSilver being you default...
The files, are .tpl, and can be opened with notepad or any html editors such as dreamweaver. Each file represents each part of the forum, ie overall_header.tpl this is your header... index_body.tpl this is the main index page with all the different sections of the forums are displayed.
Basically its easier to mod an existing template then it is to create one from scratch, as all the files names are set, along with the image file names, just make you new images and replace the old ones.
make your header, with or without the links, if you use photoshop, make sure u use slices, this would break up the design, when done, save to web, and when you selecting where to put it, you will noticed options below, images only ect... make it html and images... this will then give you the html code for the template, paste it into the overall_header.tpl, but only replace it where the html is for the file... normally at the bottom.
There are so many things, to do, just look our the old template is and basically try to make it so with yours.