My requirement is that i need tool tips with rounded corners to be put on my project. The current tool tips have a square/ rectangle border and there seems to be no skin with a rounded rectangle border.
I was curious whether is was possible to do in ASP.NET, so I did a little researching. The first thing I found is that rounded corners aren't supported in all browsers, specifically it not supported in Internet Explorer 8 or earlier. After some toying in Firefox I couldn't get the RadToolTip to display with rounded corners. I don't think it is currently possible. The best course of action may be to submit a Support ticket to Telerik to confirm that this feature isn't available currently. If it's not available you can submit a feature request to the Public Issue Tracking System below. Features are voted on my members of the Telerik user community and that information is used to help determine the roadmap for future releases.
Please note that the RadToolTip's corners are rounded by default in some of the skins and this works in all browsers, as it is done via background images and CSS.
If that is not enough for your needs the quickest way is to remove the default borders, as they are rendered with table elements and use the content wrapper and display its border. You can find a simple page that demonstrates this approach attached.
Please note that this method will only work under newer browsers (like IE9, Firefox 4, Opera 11) and under older browsers it will display regular square borders. It is also by far not the best way to modify the appearance of the tooltip.
If you need to support all browsers you will have to create a custom skin and fully customize the appearance to match you needs. More information on the RadToolTip's CSS classes is available in the online documentation (http://www.telerik.com/help/aspnet-ajax/tooltip-appearance-css-selectors.html) and can be observed in tools like firebug for fine-tuning.