You can find a list of the CSS classes used by the FormDecorator control on the following help page - http://www.telerik.com/help/aspnet-ajax/formdecorator-css-classes.html
. If you need the corresponding CSS definitions, you can get the CSS resource loaded on the page.
Please, note that the structure and names of the CSS classes and selectors has been adapted to the needs of the control itself and is generally not intended to be used as a CSS framework.
For your particular case you can check the .rfdSkinnedButton CSS class, which seems to be perfect for what you wish to accomplish. Note that using the out-of-the-box CSS resources requires that this HTML tag be part of a container with a .RadForm CSS class.
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