I recently worked on a small project designed to promote a book of Cartoon Elephants. The project had a small budget so we decided to purchase a theme. I selected http://themeforest.net/item/rythm-one-page-responsive-wordpress-theme/3672895 as its one-page responsive format was ideal for the site. However, its style was nothing like how a site devoted to a book of Cartoon Elephants should look! The answer to this was to create a child theme.
While I did create a few custom functions and other overrides, most of the work was accomplished with CSS. I also used the Tekton Pro font via typekit. The result is a fun and colorful Cartoon Elephant Book site.
The creation of the child theme points out some guidelines to consider when selecting a theme that you will later customize. First, pick a theme that has the basic functionality you need. In this case it was a responsive design with a single page approach. Next, make sure that you can create the child theme you want from the original parent theme. Since my project was fairly simple, my considerations included the ability to create the right style.