How to Get the New Gmail 'Preview' Theme

How to Get the New Gmail 'Preview' Theme:

I've been using this new theme for about a week, and it's utterly fantastic. (I prefer the Dense version.)

For the record, I've been using the gmail.com interface — not through an email client, but the in-browser version — since 2006. I've yet to find a more efficient interface for email, including Sparrow. I do, however, use Mailplane, which I've stuck with for about a year now. It helps with switching between accounts (easier than the Gmail option in the upper right), gives my mail a place in the dock, and retains the in-browser experience.