Google labs has introduced many features , Gmail themes are the part of Gmail labs. You can change the skin of Gmail to the color you like . Though these themes are fixed, you can create customized skin for your Gmail !
To change the color, first log in to your Gmail account ,
Go to settings-> and select “themes” tab
Then select “Choose your own colors”
Now a pop up opens , It will have different segments , with small drop-down icon on each segment. Here you can change Background color, Text and Border colors of your Gmail Inbox!
Try different combination of colors, All these colors given in the palette are web colors , And they appear same across all browsers and operating systems