MahApps - How to disable automatic uppercase of default button

You can override the default value by setting the property for all buttons in Window.Resources

    <controls:MetroWindow
    ...
    xmlns:controls="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
        <Window.Resources>
            <ResourceDictionary>
                <Style TargetType="{x:Type Button}" 
                       BasedOn="{StaticResource {x:Type Button}}">
                    <Setter Property="controls:ButtonHelper.PreserveTextCase" Value="True"/>
                </Style>
            </ResourceDictionary>
        </Window.Resources>
        <Grid>
             <!-- This would have normally made the text uppercase if not for the style override -->
             <Button Content="Button"/>
        </Grid>
    </controls:MetroWindow>

Omitting the x:Key setting causes the style to be applied to all buttons in this MetroWindow.

If you apply the answer of ImaBrokeDude to your App.xaml, it will work for all the buttons in any window of your project.

<Application 
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:controls="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro">
<Application.Resources>
    <ResourceDictionary>
        <Style TargetType="{x:Type Button}" 
                   BasedOn="{StaticResource {x:Type Button}}">
            <Setter Property="controls:ButtonHelper.PreserveTextCase" Value="True"/>
        </Style>     
    </ResourceDictionary>       
</Application.Resources>