GMail is now rolling out new and nice feature for millions of users. We can apply predefined theme to GMail, and believe me they all are many and nice looking.
One of my favourite is a terminal-like theme. This can make my screen very look like an AS400 dump terminal. A black and green combination, very good to impress your boss as if you are still working on AS400 but actually reading and chatting in GMail.
There are a lot of theme to apply. Whenever you login to Gmail and go to Settting, you should be able to find a new tab called Themes.