Wordpress Themes are freely available in the net. But you should use SEO Friendly Wordpress Themes for better SEO.
Themes have much to do with Search Engine Optimization. Some of them help you to get higher position for your keywords.
SEO friendly wordpress themes avoids duplicate contents from your Wordpress websites.
How seo friendly Wordpress themes avoids duplicate contents ?
Your wordpress site will contain same matter on the individual page as well as in homepage when you publish a new article. This will form duplicate content.
Friendly themes will reduce the text shown at the homepage by using a Read More link at home page. So full text of an article will only be visible at individual pages thereby avoiding redendency.
The ordering of page contents have much to do with SEO. So a fast loading simple theme with easy navigation will help you to get a good position at major search engines.