This is the result that we're looking for:

HTML:
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Test2.aspx.vb" Inherits="CRM.Test2" %>
<!DOCTYPE html>
<head runat="server">
<title></title>
<link href="/CSS/RadInput.css" rel="stylesheet" />
<style type="text/css">
body {
font-family: "Segoe UI", Arial, Helvetica, sans-serif;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
<telerik:RadTextBox runat="server" EmptyMessage="EmptyMsg" ID="RadTextBox0"></telerik:RadTextBox> enabled, empty <br /><br />
<telerik:RadTextBox runat="server" EmptyMessage="EmptyMsg" ID="RadTextBox1"></telerik:RadTextBox> enabled with text <br /><br />
<telerik:RadTextBox runat="server" EmptyMessage="EmptyMsg" ID="RadTextBox2"></telerik:RadTextBox> disabled, empty <br /><br />
<telerik:RadTextBox runat="server" EmptyMessage="EmptyMsg" ID="RadTextBox3"></telerik:RadTextBox> disabled with text
</form>
</body>
</html>
This is RadInput.css:
/*Styles the text box */
html body .RadInput_MetroTouch .riTextBox {
border-color: #e0dfdf;
color: #333 !important;
background-color: #fff;
font: 100 16px "Segoe UI",Arial,Helvetica,sans-serif
}
/* Style the disabled textbox itself */
.RadInput .riTextBox.riDisabled {
background-color: #f0f0f0 !important;
color: #333 !important;
border-color: #ccc !important;
}
/* the Telerik disabled/empty text box does not display the empty message in italics, this style is manually applied in 'populate' */
.state-disabled-empty {
font-style: italic !important;
opacity: 0.5 !important;
}
The code behind to apply the needed style to the empty/disabled text box:
During Populate(), if a RadTextBox is both disabled and empty, we manually apply the state-disabled-empty CSS class. If it has text, or if it’s enabled, we remove the class.
Public Class Test2
Inherits System.Web.UI.Page
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Populate
End Sub
Sub Populate()
RadTextBox1.Text = "Has Text"
RadTextBox3.Text = "Has Text"
RadTextBox2.Enabled = False
RadTextBox3.Enabled = False
RadTextBox2.CssClass = "state-disabled-empty"
End Sub
End Class