GMail has released two new themes that looks very nice called Preview and Preview (Dense) which you can activate in Themes tab. I have activated this two themes and i think there isn't much difference between those two (or i can't really see the difference yet).
From usability point of view, the font has been increased a bit, so it's more visible to users. In overall, they make the design quite minimalist (as usual from Google designers).
Here's the preview of the latest theme in GMail