that would be colours.css and the section would be "wrap". howver, you are not going to like it if you turn all of that blue like you have the background.
you will not be able to read any of the text and I can tell you from experience that trying to figure out where to change the text color for all those different areas is going to be a pain.
why not find a style that is closer to what you want to begin with.
also, if you make all these changes to prosilver you will have to redo them every time you update.
you are much better using a custom style and only having to update the minor changes between phpbb updates etc.
Thanks Robert you are correct sir, i tried it. I had some regulars on there tell me the white was hard on their eyes, so wanted to please them. I'll play around changing the white to something less bright and see how it looks or leave it as is if that doesn't work.