Google has added a choice of 31 different themes to its Gmail GUI. All themes retain the basic layout of the email service, but sprinkle color over the surface. There is one slightly more sophisticated theme called “Tree” that changes depending on the weather in your area. In my case, it is freezing cold (Chicago) and Gmail showed a snow theme.
It is not a dramatic improvement, but it does make the Gmail GUI, subjectively, a bit more attractive and presentable, at least if you are not a fan of the simplistic Google layout. You can browse through the themes by clicking on the Themes tab in your Gmail Setting.