Original post from Jason Cornwell, User Experience Designer (from official gmail blog)
Here's what one of the new themes currently looks like:
And in conversation view:
If you poke around you'll hopefully find a lot to like and a much cleaner, modern look but also few rough edges. In particular, some Labs features may look a little strange in the new themes. We plan to fix these issues as we roll out changes in the coming months. You can also expect some updated themes that embody the same design principles but are better suited to working in a dark environment, use a different color palette, or include the illustrations that we know many of you love to see around your inbox.