Hiya,
This is actually not possible with this particular template because the top and bottom rounded slices are fixed width.
It is quite difficult to do what you are suggesting with a transparent template because most of the methods for creating fluid layouts with rounded corners etc require that the corner slices overlap the repeating background slice - with transparency this creates a layered effect.
However there are options such as this two column version:
joomlame.joomlabear.com/
This is still fixed width of course.
Additionally are you absolutely certain you want fluid width? I'm personally not a big fan because text gets quite hard to read when it is spread out - and I currently work on a 24inch monitor.
Lastly it would be a lot easier to code a fluid width transparent template if square corners were used - would only need on repeating background slice.
I;ve done something along those lines here
stuntpixel.com/ though it isnt fluid at this stage it's a lot closer than transparent bliss.