Do you even know what CSS is? You can't just have all the different colors in one file, since they'll just override each other and mess everything up. Also, CSS isn't a template system, it's a client-side technology for specifying styles without using HTML attributes or font and center tags. As such, each layout needs its OWN CSS code, and that's if the layout is based on HTML with CSS instead of old-school HTML with nested tables and font tags.
However, not all layouts, even CSS-based, would have the same HTML, so any hack that is based around just switching out the CSS (I already mentioned one before, slightly more complex than yours but then again, it'd actually work), isn't going to cut it now that we're not talking about just switching color schemes, fonts, and a few images.
The goal here is to make them use the same template system, so the HTML of the layout can be completely changed by editing only 1 set of files instead of three, and that means more than some basic little hack. It's also a goal to use forum sessions to determine the user selected skin for the site.
This would be simple of the template systems were similar, but they aren't. phpBB uses a variant of FastTemplates, MediaWiki uses its own PHP template system that uses a PHP script as a template, and the news/story scripts use Smarty templates. Most of the template code is hardcoded into phpBB and the news/story scripts, but a PHP template can be written for MediaWiki that uses one of the other two systems.
These template systems aren't as simple as including a few files in PHP. They cache content, parse the templates for things like looping commands and variable insertions, then display the result. Smarty templates are also cached as PHP scripts to increase performance. The bad part is they all have their own interfaces that differ too much to avoid lengthy code edits.
Fortunately, MediaWiki dropped XML TAL templates mid-year and switched to using PHP scripts, so we can write a template that just uses the same template system as one of the other two scripts. Since the forum code base is larger, we'll just use the same template system as phpBB and then rewrite the news and story scripts.
The session-based skin selection is the more time-consuming part, since it means wading through a lot of phpBB specific session code. If we used all custom scripts written by me, then it'd be no problem, but editing other people's code is always time-consuming at first.
Actually, come to think of it, I've worked with phpBB sessions before, so it'll probably only take a matter of a few hours to complete the new system, with a few more to clean up.
