If I recall correctly I used firebug to get that CSS rule. It is a great FireFox add-on. IE and Chrome come with their own dev toolbars, and so does Opera. Firefox also has a built-in dev toolbar but I find it lacking compared to FireBug.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed