What actually the RadFormDecorator does is to go through the HTML elements on the page and set to those that can be decorated classes which in turn style the elements. Therefore it is normal when you perform a partial page update the FormDecorator to put such classes on the elements which were not initially decorated.
Changing the appearance of controls (e.g. RadTextBox) by overriding their current CSS with custom one is a common scenario that can be done either by setting !important to the certain selector, or by making the selector more specific.
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