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I encourage you guys to consider adding multiple theme support for vanilla.
For example, one theme for the website, one theme for embed, and one theme for mobile. That would be great for Facebook integration.
We've got one theme for the website and one theme for mobile. The embed one is on the roadmap but it's a bit more involved.
We don't know that a site is embedded at the right time as far as enabling themes goes so we need to program the ability to serve the site from a different set of urls. This is a lot of work and will get the people who are queasy about canonical urls all fussy.