Creating new sites with iWeb is not a brilliant idea since the application is no longer being developed or supported.
Creating your own "theme" or design using iWeb isn't too good an idea either since this usually involves a whole bunch of PNG images which is one of the reasons that iWeb created sites take so long to load in the browser. If you were going to design a business site in iWeb it should be as simple as possible with absolutely no strokes, frames, borders, drop shadows, translucent overlays etc.
The main advantage to using a master sylesheet for the whole site is that the browser only has to download it once. This is in contrast to an iWeb site where all the files for the page have to be downloaded for each one. Not a very efficient way to create a website even although it does allow those with little or no knowledge to create sites.
I'm not putting down iWeb for this since it is actually a lot better than most applications of its type and certainly superior to Wordpress which is probably one of the most popular ways to create websites but also one of the worst.
Nowadays, HTML is very simple to learn since all the hard work is done in the CSS stylesheet. There are literally thousands of free stylesheets to found on the internet and a quick search is bound to turn up something that you like.
Here's a quick example of one that's been modified only slightly using the freeware Fraise...
http://www.ezmacwebdesign.com/Demo/free/
The header image and few other styles were changed to produce a design for a total cost of $0 - apart from a little of my time.