Background does not change of button C# WPF

Question

I am trying to change style on mouseover.

My Code is:

Answer 1

Edit: As Rohit pointed out in the comments,

This won't work on Windows 8 because of some changes in PresentationFramework where default template of button is declared. ... In Windows 7 default template of button doesn't have that ControlTemplate trigger. That's why your posted code works fine on Windows7 but it won't work on Windows 8 and higher.

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Rohit's answer about DependencyProperty Precedence being the cause is correct, but there's a much simpler way of fixing it than overwriting the Button's Template.

If you review the DependencyProperty Precedence List, you'll notice that properties set in the have a higher precedence than Triggered properties, which is why your button will always use the background you have defined in your tag.

If you move the Background property to the