.skin file takes precedence over styles applied to every HTML element
Suppose you have a definition for the TextBox server control in the .skin file:
<asp:Textbox Runat="server"
ForeColor="#004000"
Font-Names="Verdana"
BackColor="#ffffff"
Font-Size="X-Small"
BorderStyle="Solid"
BorderWidth="1px"
BorderColor="#004000"
Font-Bold="True" />
CSS file
INPUT {
background-color: black;
}