Themes/MaterialDesignTheme.Font.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
 
  <FontFamily x:Key="MaterialDesignFont">pack://application:,,,/MaterialDesignThemes.Wpf;component/Resources/Roboto/#Roboto</FontFamily>
 
</ResourceDictionary>