Trying to override the Menu brushes in Callisto

App.xaml

<Application x:Class="Test.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>

                <!--
                    Styles that define common aspects of the platform look and feel
                    Required by Visual Studio project and item templates
                -->
                <ResourceDictionary Source="Styles/Brushes.xaml" />
                <ResourceDictionary Source="Themes/Generic.xaml" />
            </ResourceDictionary.MergedDictionaries>

            <!--  Application-specific resources  -->
        </ResourceDictionary>
    </Application.Resources>
</Application>

Brushes.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:controls="using:Money.Controls">

    <!-- overrides for default controls : This seems to work -->
    <SolidColorBrush x:Key="HyperlinkForegroundThemeBrush" Color="#308EC9" />
    <SolidColorBrush x:Key="HyperlinkPointerOverForegroundThemeBrush" Color="#5EB7F1" />
    
    <SolidColorBrush x:Key="ComboBoxItemPointerOverBackgroundThemeBrush" Color="#A5A6A8" />
    <SolidColorBrush x:Key="ComboBoxItemSelectedBackgroundThemeBrush" Color="#393A41" />
    <SolidColorBrush x:Key="ComboBoxItemSelectedPointerOverBackgroundThemeBrush" Color="#A5A6A8" />
    
    <SolidColorBrush x:Key="ToggleSwitchCurtainBackgroundThemeBrush" Color="#308EC9" />
    <SolidColorBrush x:Key="ToggleSwitchCurtainPointerOverBackgroundThemeBrush" Color="#5EB7F1" />
    <SolidColorBrush x:Key="ToggleSwitchCurtainPressedBackgroundThemeBrush" Color="#5EB7F1" />

    <SolidColorBrush x:Key="ListViewItemSelectedPointerOverBackgroundThemeBrush" Color="#00FFFFFF" />
    <SolidColorBrush x:Key="ListViewItemSelectedPointerOverBorderThemeBrush" Color="#80FFFFFF" />
    <SolidColorBrush x:Key="ListViewItemSelectedBackgroundThemeBrush" Color="#40FFFFFF" />
    
    <!--  brushes for list items -->
    <SolidColorBrush x:Key="CustomListItemHighlightBrush" Color="#80FFFFFF" />
    <SolidColorBrush x:Key="CustomListItemSelectedPointerOverBrush" Color="#A5A6A8" />
    <SolidColorBrush x:Key="CustomListItemCheckBrush" Color="#1A1C21" />
    <SolidColorBrush x:Key="CustomListItemCheckHintGlyphBrush" Color="White" />

    <!-- This doesn't seem to work -->
    <SolidColorBrush x:Key="MenuItemPressedBrush">#1A1C21</SolidColorBrush>

    <!-- this doesn't work    
    <ResourceDictionary.ThemeDictionaries>
        <ResourceDictionary x:Key="Default">
            <SolidColorBrush x:Key="MenuItemPressedBrush">#1A1C21</SolidColorBrush>
        </ResourceDictionary>
    </ResourceDictionary.ThemeDictionaries>-->
    
</ResourceDictionary>