Hi Dominik,
It appears the .LabelCls was an extra property we had added, although some refinements were made with the v1.0 build and it was removed with a preference to use the .ItemCls property.
The following demonstrates using the .ItemCls property to define a specific label style.
Example
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Example</title>
<style type="text/css">
.required .x-form-item-label {
font-weight : bold;
color : red;
}
</style>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:FormPanel runat="server" Title="Example" Height="185" Width="350" Padding="5">
<Items>
<ext:TextField runat="server" FieldLabel="My Label" ItemCls="required" />
</Items>
</ext:FormPanel>
</form>
</body>
</html>