You can do it by changing the Button's ControlTemplate. Copy those style, brushes and ... to you resource dictionary then change the values you want.
To change the border thickness find the following code and make changes you want:
...
<Border
x:Name="Border"
CornerRadius="2"
BorderThickness="1" //CHANGE THIS VALUE
Background="{StaticResource NormalBrush}"
BorderBrush="{StaticResource NormalBorderBrush}">
<ContentPresenter
Margin="2"
HorizontalAlignment="Center"
VerticalAlignment="Center"
RecognizesAccessKey="True"/>
</Border>
...