Your content pages don't have any mark-up that is sent to the client (other than the mark-up you add yourself), they are simply a server-side container for your controls. If you want to centre your content you'll need to wrap your content placeholder in a div with the appropriate css
#content{
position: fixed;
right: 0;
left: 0;
margin-right: auto;
margin-left: auto;
min-height: 10em;
width: 90%;
}
On your master page
<div id="content">
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</div>