The search engine otpimization strategy of theming a web site is more common sense and usability than trickery. The basic premise behind the strategy of theming a website is that each section of the website must be relevant to the other section, and only the sections that are relevant to each other are linked together.
One of the biggest incentives to theme a site is that the some of the search engines use themes as part of their algorithm for inclusion into their database of websites. Not only do they use themes as a factor for inclusion, but they also use themes as a factor for deciding which web site ranks higher.
One of the reasons that many webmasters and search engine optimizers do not use themes is because to theme a site it requires that you construct the site from the beginning with the themes in mind, or you do a total site reconstruction or renovation. Although many people do not opt to go for the themed approach, it is one of the most effective and safest ways to optimize a site.