Now Gmail has an option to customize its default theme. You can change colors according to your likes and dislikes.
Just go to Settings->Themes and click "Choose your own colors" and select your favorite colors for Gmail's background, links, text messages, navigation links and more.
Once you are done coloring your Gmail click save and your choice is save and applied automatically.
In case you dont like your theme you can easily Reset the theme or choose from several pre-defined (not so good) themes.