You can use an inherited style with no key reference:
<Grid>
<Grid.Resources>
<!-- Definition of default appearance of a button -->
<Style TargetType="Button" x:Key="Default">
<Setter Property="Background" Value="Red"></Setter>
<Setter Property="FontFamily" Value="Segoe Black" />
<Setter Property="HorizontalAlignment" Value="Center" />
<Setter Property="FontSize" Value="32pt" />
<Setter Property="Foreground" Value="#777777" />
</Style>
<!-- Define general style that based is base on the default style of the button without a key reference-->
<Style TargetType="Button" BasedOn="{StaticResource Default}"/>
<!-- In sub style override the properties you need -->
<Style BasedOn="{StaticResource Default}" TargetType="Button" x:Key="SubButton" >
<Setter Property="FontSize" Value="8pt" />
</Style>
</Grid.Resources>
<Button Content="Main" Height="51" HorizontalAlignment="Left" Margin="154,72,0,0" Name="button1" VerticalAlignment="Top" Width="141" />
<Button Content="Sub" Style="{StaticResource SubButton}" Height="51" HorizontalAlignment="Left" Margin="154,162,0,0" Name="button2" VerticalAlignment="Top" Width="141" />
</Grid>
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