Changing the Background Colors - Confused over css, bg.png

[catalyst]

I am working to build out a refresh of my current website (www.securitycatalyst.com) with a "new and improved" version using streamline. I'm building/staging/testing at www.securitycatalyst.info.

I would like to change the background colors (to something lighter); however, I would like to keep the "color scheme" in terms of contrasts, just move out of the shades of grey. I have opened and found styles.css - and understand that it calls bg.png. I have opened bg.png in photoshop - but I'm not sure where or how to modify the file.

I would be happy to recreate a file using a different color scheme, but outside of not being sure where to start, I don't fully understand how to measure the current gradients so I can keep the same look and feel. Is there a specific approach I should follow to create (re-create) bg.png with different colors?

I hope I am explaining this well. While I can muddle through PHP and other elements - this is an area where I lack the level of experience, perhaps, to explain. Bottom line: how do I lighten/change the colors of the background, in the same style that exists?