Hi Hery,
The most likely reason for this problem is a global style on the page (for example for TD / TR / UL / LI tags) that interferes with RadWindow's CSS skin. Please check your page and if you cannot locate the problematic style, send a live Url to a page where the problem could be reproduced. I will check it and get back to you right away.
Kind regards,
Georgi Tunev
the Telerik team
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