While some have complained about the changes in some of the functionality of Gmail, I think the feedback on the new look, and the new Themes that are available has largely been positive. At least I’ve not heard any complaints.
It’s actually pretty cool that some of the new Themes, as well as few of the Classics, are provided with multiple variants, based either on day or time of day. The above screen shot, of one of my empty Gmail accounts, shows a variation on the new Mountains HD Theme. You can tell which of the new or updated Themes is provided with variations by looking at its thumbnail on the Themes page.
I’ve not come across any listing of all the variations of the multi-variant Themes, showing either the number of variants, or the images themselves, but here are the changes I’ve noticed so far.