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I think sitemesh and themes are completely different things. Themes allow you to include in a web page "theme" specific stuff (for example, you could have several different gif files for a logo, a different gif for each "theme").
sitemesh, on the other hand, extracts sections of html out of a source web page and embeds that html in another page. This allows you to build wrappers, or "decorators" for basic content-only oriented web pages (which may contain "theme" specific data).