As far as I understood your requirements:
You have two Buttons
("Login" and "Register") which need to be visually reduced (should not use visual states) and which have their own "text style" each (but maybe sharing a base style)? Is this correct?
If this is the case I recommend two different Button Styles
(maybe sharing the same base style):
<Button x:Name="loginBtn" Content="Login"
Style="{StaticResource LoginButtonStyle}"/>
<Button x:Name="registerBtn" Content="Register"
Style="{StaticResource RegisterButtonStyle}"/>
And your styles:
<Style x:Key="PlainButtonStyle" TargetType="Button">
<Setter Property="FontFamily" Value="Segoe UI Light"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Border BorderBrush="Orange" BorderThickness="1" CornerRadius="6">
<TextBlock Text="{TemplateBinding Content}"
Padding="{TemplateBinding Padding}"
HorizontalAlignment="Center"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="LoginButtonStyle"
BasedOn="{StaticResource PlainButtonStyle}" TargetType="Button">
<Setter Property="FontSize" Value="10"/>
<Setter Property="Padding" Value="0,10,0,10"/>
</Style>
<Style x:Key="RegisterButtonStyle"
BasedOn="{StaticResource PlainButtonStyle}" TargetType="Button">
<Setter Property="FontSize" Value="40"/>
<Setter Property="Padding" Value="0,20,0,20"/>
</Style>
Properties like FontSize
and FontFamily
are inherited.