As far as I can see from your screenshot the problem is caused by usage of global styles on your page.
Please, note that the RadControls are actually complex HTML and they are formed by standard HTML elements which are styled in a certain way. That is why when using them as well as any other third party controls, it is strongly recommended to avoid using global styles because they can override the default settings of the elements which form the third party control and to ruin their appearance as in your case.
This being said, please remove the global styles and set the desired CSS to your custom HTML elements by assigning a class name to them - in this manner you will style your page as you desire without affecting RadWindow or any other RadControl on your page.
All the best,
the Telerik team
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