<Application x:Class="WpfApplication.App"
             xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
  <Application.Resources>
    <ResourceDictionary>
      <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />
        <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Fonts.xaml" />
        <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Themes/Light.Blue.xaml" />
      </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
  </Application.Resources>
</Application>

<mah:MetroWindow x:Class="WpfApplication.MainWindow"
                 xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:mah="https://metro.mahapps.com/winfx/xaml/controls"
                 Title="MainWindow"
                 Height="600"
                 Width="800">
  <Grid>
    <!-- your content -->
  </Grid>
</mah:MetroWindow>