These are usually very carefully crafted custom templates, but SOMETIMES they are clever "tweaks" to the css.

When I see a site I like, I copy their css and (using firefox tools), "apply" their css to my site to see what happens.

Sometimes, they use a "free" template, and then customize the css.

I am doing this with the apple_zen template.

If you look at:

(sorry, site offline) and load the Apple Zen template
compare it with:

www.callmecupcakes.co.uk
... yes, both are Apple Zen core templates!