page design within master page

asp.net web designing is same as html designing.

//This is an asp.net control:

<asp:textbox id="text" runat="server" xmlns:asp="#unknown" />

//This is html tag

<input type="text" />

-- CSS --
// Css class that we will be implementing in both asp.net control and html tag

.inputCss{
color:red;
}

// Implementing CSS Class

<asp:textbox id="text" runat="server" cssclass="inputCss" xmlns:asp="#unknown" />

<input type="text" class="inputCss" />

you can also write simple class attribute for asp.net control.

<asp:textbox id="text" runat="server" class="inputCss" />

Suggestion:-
My suggestion to you is learn html designing because all ASP.Net control first renders in HTML tags. The way you can check this is open any page of asp.net in your browser and do inspect element or press (Ctrl+Shift+I) in google chrome or F12 in firefox you will see that all asp.net control will be rendered to html tags.

Hope this helps you a bit.