Google/Gmail has publicly released their new Themes feature and I have to say it looks kickass. This has been a long time coming considering third parties have created this functionality for the old Gmail and now that it's released as an internal feature, the new Gmail now looks beautiful as it is functional. Customizable themes just make the mailbox that much more personal.
Although this isn't a full opening of their color schema, it's just one step short of it. Right now you can only select pre-made templates that Google provides. I can't wait until Google releases a fully customizable color schema. Maybe even a UI layout modifier, but I think that's asking for a little bit too much.