The header is centered on the screen with a lot of grey around it. Underneath, the page title followed by the image that was embedded in the first paragraph. Then the text centered...
Do you mean that it's supposed to look like that, or that it's coming out that way although you don't want it to?
Each of the features you describe should be explicitly spelled out in the CSS. Text alignment, padding, top and bottom margins, header color, background color, et cetera. In fact, it's a good idea to make up some boilerplate that you put in every stylesheet, so you'll have your starting position.
Leaving these things out will not affect validation, because they're not errors. They simply mean that each browser will use its own defaults-- and that's what you're trying to override. Browser defaults are good for individual user choices, notibly their preferred font and size for regular text. (PLEASE don't change these, unless you have a very narrowly defined target audience and don't mind annoying the others.) Everything else is for you to personalize.