Else, you may go to settings to see the ‘Themes’ Tab there.
There are lots of themes being provided to choose from, I tried few of them , this is ‘Terminal’ theme which turns it like ASCII theme,
Frankly, I am comfortable with the default look or the ‘Classic’ Theme as most of the themes are too fancy or dark colored, I like clean and brighter look with lighter shades.So I would prefer to choose lighter colored Inbox instead of full themes. They should have left the ‘Gmail Beta’ Logo as it is instead of changing them according to the Theme.
These themes are being rolled out to everyone in next couple of days, so if you are not seeing them, check them again later. Recently Windows Live Hotmail also updated with some new themes, now Gmail comes with its themes.