For discussion and questions related to CMS Specific templates and stylesheets (CSS), and themes. or layout issues. This is not a place for generic "I don't know CSS issues"
The pagewrapper div in some of the default templates is the container around the whole site, keeps it a certain size etc. as for the rest of your question you may need to explain in a little more detail...
When you target a div in CSS it doesn't do anything that you don't tell it to do so it doesn't override anything, I think you are kind of confused about viewport, which is the area inside the browser where the website is, and canvas, which is the whole basically the same thing but used in a different manner when speaking about web sites...