There are several ways to achieve this:
1) Use CSS to change the look for all the input controls. Just put "input[disabled] { border:solid 1px red; }" in your CSS file.
2) Using ASP.NET skins, in the Skin file you could write:
<asp:TextBox runat="server" SkinID="disabled" CssClass="disabled"></asp:TextBox>
This would set the disabled CSS class only for that TextBox
3) Manually assigning a CSS class to only the TextBoxes you want disabled, like:
<asp:TextBox runat="server" ID="x" Enabled="false" CssClass="disabled"></asp:TextBox>
4) Create your custom control that inherits from TextBox and configure it that way.