Reg: Silverlight 4.0 Applying themes dynamically at runtime through code

Hi all,
 I am working on silverlight 4.0. I am facing a problem with applying dynamic themes to my application. Please go through below and send me your solutions -

Problem:
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In my application I have three ResourceDictionary files -

1. Default.xaml
2. Theme1.xaml
3. Theme2.xaml

In all above files I 2 style keys mentioned below and three buttons.

1. x:Key="ButtonStyle"
2. x:Key="LayoutRootGridStyle"

I have given x:Key="ButtonStyle" to only one button. My requirement is if click on three buttons all the buttons and text boxes styles are changing. but the both x:key styles are not changing. Please tell me the problem and solution asap.

In my main application I am using may borders and grids if I give the styles with out key the style applying to all. I don't want that. I just want to apply styles to given key only.

Thanks in advance.