In order to keep the rounded corners, you can set Skin="" and EnableEmbeddedBaseStyleSheet="false" to the RadInputManager. However, by doing this, you will lose the error state of the textboxes. The textboxes can be styled either by RadFormDecorator or by RadInputManager, but not both.
RadFormDecorator only styles textboxes, which have no CSS class. That's why you need to completely disable the RadInputManager skinning in your case, so that the textboxes do not receive CSS classes.
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