Hi Andrew,
This is going to be pretty complex but its certainly achievable. The
template was really designed to be a full screen theme and you may find changing scroller to suit your purposes a little easier.
The main sticking point will be the left panel which is fixed in place.
You'll need to create a container around the whole of the template and then give the container the desired width plus margin:0 auto. So add your container just after the start of the body tag and a closing div at the end before the closing body tag.
That'll place everything in the template within the container but you'll find the left panel more than likely sticking in its place.
There will be a few more tweaks needed here but they should be easy enough to see what needs to happen.
.. Actually I just tried it on a demo version and it does work nicely. You'll have to play with the background image though and perhaps change the bg image in the theme from body to scroll-wrapper or scroll-inner ... which ever works best.
Let me know how it goes.