Any user working on a simple desktop is aware of the themes of the computer. The themes which are used on the computer are simply related the colors and the background of the desktop, but as far a the themes in search engine optimization is concerned it is actually related with subject matter of the website based on which it has been made. The theme of the website can be any on real estate, fashion, health and so on. The major search engines like

Google and Yahoo rank the websites based on the themes. This is because every website has been divided into various categories according to its themes.
Page ranking of a website depends on the correct and sole usage of the themes. Like if a site is made of health theme, then every aspect and page of the site must talk about health and not on any other topic, doing this a website can attain a good page ranking by the search engines. Making it clearer, we can say that health related sites should only talk about health related issues and if the webmaster wants to increase the page ranking of the website then the rest of the pages should contain the interesting and different topics on health, so that the search engines find the website interesting and the visitors.

Any webmaster working on the themes on his website should maintain a rhythm on his site and hire a good SEO company to promote each page of his website and get better page rankings for his website.