Kenneth is correct, your Style overlays ontop of the original background
I'm not sure if which of these you need but if you force their background to be from the parent then it should resolve the issue.
<border x:name="ButtonBorder" xmlns:x="#unknown">
CornerRadius="10,10,10,10"
BorderThickness="4,4,4,4"
removed="#00000000"
BorderBrush=" #FF8F8F8F"
RenderTransformOrigin="0.5,0.5"
Background="{TemplateBinding Background}"
></border>
or
<contentpresenter x:name="ButtonContentPresenter" xmlns:x="#unknown">
VerticalAlignment="Bottom"
HorizontalAlignment="Center"
Background="{TemplateBinding Background}"
/> </contentpresenter>
You shouldn't need to change the button XAML, just the control template.