I am sure you all know ThemeForest marketplace from Envato. If not, than you should check it out.
There you can find WordPress themes, Joomla themes, CSS templates and a bunch of other goodies for prices strating from as low as $3!
What you might not now is that Collis Ta'eed is offering his know how of how to design a WordPress theme from A to Z in his book called "How To Be A Rockstar WordPress Designer". Not only you'll be able to understand a WordPress theme structure and what are the steps of modifying a WordPress theme or build one from scratch, but you will get 3 awesome WordPress themes (blog, portfolio and general themes) and you can use them on your own site(s) and you will know how to modify them to make the themes unique!
If you ask me, every blogger out there should read Collis Ta'eed's book because this can be really a bible for every blogger and will save you a lot of money spent on designers every time you want to make a change on your website.
Check out How To Be A Rockstar WordPress Designer