Subscriptions: allow site owners to control the design of the emails

Although we can't allow too many changes to the original template since the emails are sent from WordPress.com, it would be nice if we could automatically grab color values defined in the site's theme (things like [default background color](https://codex.wordpress.org/Function_Reference/add_theme_support#Custom_Background), [default header text color](https://codex.wordpress.org/Function_Reference/add_theme_support#Custom_Header)), and use these in the notification emails.