The AjaxControlToolkit project is not related to our software and therefore we do not support custom implementations of its skins. This also means that you cannot apply a RadControl's skins to it directly.
Nevertheless you can inspect the colors from the skin you want to mimic and use them in your CSS for the Accordion control, essentially creating your own AjaxControlToolkit skins. For example, some of the colors that our Windows7 skin applies are:
#dde9f7 - light blue
#c3dcfc - active color
#b8cbde - border normal
#c1dbfc - border hover
Note that RadFormDecorator is supposed to decorate Buttons, Checkboxes, FieldSets, Labels, Headings etc (you can find the full list of supported elements here and here, it is essentially the standard form HTML elements). Therefore if you have some of these elements in the Accordion and they do not have a CSS class applied and your page uses RadFormDecorator they will be decorated.
the Telerik team
Explore the entire set of ASP.NET AJAX controls we offer here
and browse the myriad online demos
to learn more about the components and the features they incorporate.