It is almost universally agreed by search engine optimization (SEO) professionals that Google and probably the other search engines are using themeing to evaluate the relevance of incoming and outgoing links. Many also agree that themeing is used to measure the relevance of both on and off page content as part of the search algorithm. The question for most SEO experts is to what degree that themeing is indeed being employed by the search engines.
Themes are thought to be used by the search engines to determine search engine rankings. With that in mind, website owners should be conscious of maintaining the theme relevance on their sites. With the rise of hub sites and authority sites to prominence in Google, the need to maintain relevance of theme became much more important.
In order to properly evaluate hub and authority sites, both on page and off page content factors have become much more important than ever before. Incoming links gain more value if they arrive from similarly themed web pages. Sending pages containing entirely irrelevant content will be awarded less weight.
The methods used in the search algorithms to determine relevance of theme are not publicly known, but the implications of the theme basis in the algorithms are obvious in the search results.
More than ever before, search engines are developing techniques to determine the themes and topics of websites, and their individual pages. Failure of a webmaster to pay attention to the changes will cause the site in question to drop far down in the search results.
Web pages that are relevant to the search are what a web searcher wants displayed in any search engine results. The search engine algorithm designers understand that desire on the part of their users. Failure to provide good search results will send users to another search engine. The search engines take themes and relevance seriously. So should you.
If you want to turn your site into the very important hub and authority sites, attention to page and site wide themes is a must.
Google Optimization 
