WPF style is not applied at runtime (but works in designer)

I have this code in a WPF window:

<Window.Resources>
    <Style x:Key="MahappsStyle">
        <Style.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/Colors.xaml" />
                    <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/Blue.xaml" />
                    <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/BaseLight.xaml" />
                </ResourceDictionary.MergedDictionaries>
            </ResourceDictionary>
        </Style.Resources>
    </Style>
</Window.Resources>

      

        
        
        
      

    

The idea is to include external styles in the dictionary for individual elements in my application. For example, it should work by applying the "MahappsStyle" style to an element called "HamburgerMenu":

<mahapps:HamburgerMenu x:Name="hamburgerMenu" Style="{StaticResource MahappsStyle}"
                    DisplayMode="CompactOverlay">
</mahapps:HamburgerMenu>

      

        
        
        
      

    

But this approach only seems to work in the designer, not at runtime. What am I missing? Is there any other way to set MergedDictionaries to a single item?

UPDATE. ... Found a way to do it. First you need to create a Mahapps.xaml application with the following content:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:mahapps="http://metro.mahapps.com/winfx/xaml/controls"
                    xmlns:local="clr-namespace:Promt.Desktop">

    <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/Colors.xaml" />
        <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/Blue.xaml" />
        <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/BaseLight.xaml" />
    </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

      

        
        
        
      

    

And then it can be applied to one element:

<mahapps:HamburgerMenu>
            <mahapps:HamburgerMenu.Resources>
                <ResourceDictionary Source="pack://application:,,,/Promt.Desktop;component/Styles/Mahapps.xaml"/>
            </mahapps:HamburgerMenu.Resources>
</mahapps:HamburgerMenu>

      

        
        
        
      

    

I am very disappointed that the ResourceDictionary cannot contain an x: key property. If someone knows a different approach - write it.

UPDATE2. An even better solution from Evk (based on Light's answer).