using AjaxControlToolkit;
...

namespace Core35
{
[TargetControlType(typeof(Control)]
[ClientScriptResource("Core35.FocusBehavior", "focusBehavior.js")]
public string HighlightCssClass
{
get { return GetPropertyValue("HighlightCssClass", ""); }
set { SetPropertyValue("HighlightCssClass", value); }
}

[ExtenderControlProperty]
public string NoHighlightCssClass
{
get { return GetPropertyValue("NoHighlightCssClass", ""); }
set { SetPropertyValue("NoHighlightCssClass", value); }
}
}

Type.registerNamespace('Core35');

Core35.FocusBehavior = function(element)
{
Core35.FocusBehavior.initializeBase(this, [element]);

this._highlightCssClass = null;
this._nohighlightCssClass = null;
}

Core35.FocusBehavior.prototype =
{
initialize : function()
{
Core35.FocusBehavior.callBaseMethod(this, 'initialize');
this._onfocusHandler = Function.createDelegate(this, this._onFocus);
this._onblurHandler = Function.createDelegate(this, this._onBlur);
$addHandlers(this.get_element(),
{ 'focus' : this._onFocus,
'blur' : this._onBlur },
this);
this.get_element().className = this._nohighlightCssClass;
},
dispose : function()
{
$clearHandlers(this.get_element());
Core35.FocusBehavior.callBaseMethod(this, 'dispose');
},
_onFocus : function(e)
{
if(this.get_element() && !this.get_element().disabled)
{
this.get_element().className = this._highlightCssClass;
}
},
_onBlur : function(e)
{
if(this.get_element() && !this.get_element().disabled)
{
tis.get_element().className = this._nohighlightCssClass;
}
},
get_highlightCssClass : function()
{
return this._highlightCssClass;
},
set_highlightCssClass : function(value)
{
if(this._highlightCssClass != value)
{
this._highlightCssClass = value;
this.raisePropertyChanged('highlightCssClass');
}
},
get_nohighlightCssClass : function()
{
return this._nohighlightCssClass;
},
set_nohighlightCssClass : function(value)
{
if(this._nohighlightCssClass != value)
{
this._nohighlightCssClass = value;
this.raisePropertyChanged('nohighlightCssClass');
}
}
}
//Optinal descriptor for JSON serialization
Core35.FocusBehavior.descriptor = {
properties : [ {name: 'highlightCssClass', type : String},
{name: 'nohighlightCssClass', type : String}]
}

//Register the class as a type tha inherits from Sys.UI.Control
Core35.FocusBehavior.registerClass('Core35.FocusBehavior', Sys.UI.Behavior);

<asp:TextBox ID="TextBox1" runat="server" EnableTheming="false" />
<asp:FocusExtender ID="FocusExtender1" runat="server"
TargetControlID="TextBox1"
NoHighlightCssClass="LowLightTextBox"
HighlightCssClass="HighLight" />