Hi Umut,
I have just been reviewing this request for help. First of all If you change the background image in the Lambda theme (Page Background settings page, from memory it has a long list of images to choose from). Then give me a link to your site. You can send me a message if you do not want to post it here. I will then take a look at your site and be able to tell you how to write the CSS to add the colour you want.
Does that sound a reasonable solution?
Cheers
Mary