I would suggest using master pages to control the overall layout of your site, so you don’t need to repeat the layout on each page. You don’t need to do anything special in the CSS file for the main page account, but you must add the link to the CSS file in the header of the main page so that it is transferred to all pages using the page wizard.
Main page title:
<head runat="server">
<link rel="Stylesheet" href="~/pathtocssfile/cssfilename.css" type="text/css" />
</head>
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