Putting into production
If you use several of elements in your app, you'll get a lot of network requests (one for each HTML import), and that's not necessarily good for performance (until we all start using Http2). It's therefore often necessary in today's web applications to concatenate Web Components into a single file.
Concatenating Web Components
The Polymer authors have developed a handy tool for that purpose that's called Vulcanize. It's a command line tool that easily integrates with your build tool of choice, that recursively pulls in all your imports, flattens their dependencies and spits out a single file.
In the future
We have a specific build tool in the works that will leverage Vulcanize and include CSS Variables post-processing. It will be released in a few weeks, stay tuned!