Themes are considered to be an important factor when it comes to 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 has become so much more important.
Links play a significent aspect of theme building, Incoming links gain more value if they arrive from similarly themed web pages. Outgoing links to other similar themed sites can also be very positive as well for your website, the important issues when it come to links are as follows:
The Age of links
Quality of domains sending links
Quality of pages sending links
Anchor text of links
Link quantity/weight metric (Pagerank or a variation)
Subject matter of linking pages/sites
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.
By getting links from and sending links to other websites that are on a similar theme to your website is very positive from a search engine ranking perspective.