The theme is a graphic template which can be applied to your website very easy. The installation of a theme is simple.
Choosing a WordPress Theme by David A. Cox
Drupal Tutorials #4 - Installing a Theme by LevelUpTuts
Picking a Joomla Theme/Template by WeEnvisionSuccess's channel
The website’s theme has to be SEO user-firendly for you, from here you can buy user templates and seo friendly at low prices, here also you can find 10-13 themes pachages, at 30 $ and one single template costs 40$.
Mojomarketplace- offers a community of professional designers and developers. Browse premium products to build, brand & grow your website.
Themeforest- marketplace for premium website templates, including themes for WordPress, Magento, Drupal, Joomla, and more. Create a website, fast.
Eleganthemes.com - Amazingly beautiful Premium WordPress Themes coupled with advanced functionality and awesome support.