Round the corners of a button in Window.Resources WPF

Question

I am very new to WPF. I've familiarized myself with ControlTemplate, ContentPresenter, and some other pieces, but I am struggling to create a button with rounded corners as defined from Window.Resources (or potentially a separate style file).

Anyways, when I add this to a <Button/> tag, I get a button with rounded corners:

<Button.Resources>
    <Style TargetType="Border">
        <Setter Property="CornerRadius" Value="5"/>
    </Style>
</Button.Resources>

However, when I try to include it up in the Window.Resources the button will not apply the border style:

<ControlTemplate x:Key="roundbutton" TargetType="Button">
    <Grid>
        <Rectangle Fill="{TemplateBinding Background}" Stroke="{TemplateBinding Foreground}"/>
        <Border CornerRadius="5"/>
        <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
    </Grid>
</ControlTemplate>

This doesn't look right to me, but I guess I don't know where to add the specification of CornerRadius such that it will apply to the button. I did see this post, How to create/make rounded corner buttons in WPF? but it's a little over my head. I just want to round all the buttons in my application!