That usually depends on the author of the webpage. I think MacNN just have it set up that way.
However, there is a way to override it. In OmniWeb, choose Site Preferences (command-option-comma). Click the PAge Appearance tab, and from the Page Colors popup menu choose Custom. Set them up how you like them, then turn on the box Override Page Styles.
If you use a different browser, tell us which one and we'll try to help
Amorya