What technology based site is it? If it is in ASP.NET, then the chances are there must be a master file for each page. Locate the master page by having a look at the extension of the page (it is .master). Then locate if any background property present. If it is not there, it means that the property is present in the CSS of the site. Search the CSS and look for the background property and it will be easy to modify it. You can get color codes (hex codes) for many of them online.