At last year’s SenchaCon we demonstrated an exciting new concept theme called Neptune. While it was only a screenshot on a slide, it caused quite a stir and has been one of the most requested features ever since.
I don't know what appeared on that slide but I think the screenshots in this blog post are what most people think of when you mention the Neptune theme:

http://www.sencha.com/blog/ext-js-4-...r-more-stable/

To quote from that post:

This builds on the great looks and clean design of the current theme, but brings a whole new level of visual polish to your applications.
I would argue that the theme previewed in 4.1-pr1 is not really the Neptune theme people were asking for. That's not to say it isn't welcome, I just don't think it's fair to describe it as the Neptune theme.