Gmail is no doubt one of the best web email system on internet these days, and with the newly introduced gmail themes including gmail preview and others it has become more simple and provides the sleek interface. On the other hand there are lot of colorful themes as well in gmail which makes it full of life colors and there some geeky themes as well like android theme for gmail geek users, I my self love the android green colored theme in gmail.
You can access the gmail themes in gmail under mail settings >> themes.
Moreover there are many new and old themes to choose from as shown in the snapshot below.
Under gmail theme you will also find last theme named random in the last row, this means if you select this theme option in gmail, every time you login to your gmail account you will see a new theme applied in your gmail account already and moreover the theme applied will be selected on random basis out of all the existing theme options as shown in the image above. You can see the random theme snapshot with question mark in the image which resembles any theme in gmail will be selected as shown in the image below.
Your preferences for the theme selection will be saved once you select any theme under gmail themes section