When you make an SSRS report with a non-white background, you may initially notice some white around the edges of the report. This MSDN forum post helped explain why this occurs. When you set the background color property on the report body through the wizard-like UI (shown below), you are setting the background color of the report body only. In my example, I have chosen the color WhiteSmoke to provide a light gray background and remove the harshness of the plain white.
This does not change the background color around the report (below and to the right) in the case that the report does not take up your entire screen. So you end up with something like this:
There is another property you must set for the entire background around the report to be the same color. You can access it in the report properties grid (select the report and hit F4).
Once I change the BackgroundColor property on the report under the Page group to WhiteSmoke, I get a complete background of light gray. The white around the sides is gone.