App.xaml v2

<Application
    x:Class="App2.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="using:App2" >

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.ThemeDictionaries>
                <ResourceDictionary x:Key="Dark">
                    <SolidColorBrush x:Key="MyForegroundBrush" Color="{Binding Source={ThemeResource PhoneAccentBrush}, Path=Color}"/>
                </ResourceDictionary>

                <ResourceDictionary x:Key="Light">
                    <SolidColorBrush x:Key="MyForegroundBrush" Color="{Binding Source={ThemeResource PhoneAccentBrush}, Path=Color}"/>
                </ResourceDictionary>

                <ResourceDictionary x:Key="HighContrastBlack">
                    <SolidColorBrush x:Key="MyForegroundBrush" Color="Black"/>
                </ResourceDictionary>

                <ResourceDictionary x:Key="HighContrastWhite">
                    <SolidColorBrush x:Key="MyForegroundBrush" Color="Black"/>
                </ResourceDictionary>

                <ResourceDictionary x:Key="HighContrastCustom">
                    <SolidColorBrush x:Key="MyForegroundBrush" Color="Black"/>
                </ResourceDictionary>
            </ResourceDictionary.ThemeDictionaries>
        </ResourceDictionary>

        <SolidColorBrush x:Key="PivotHeaderForegroundSelectedBrush" Color="DarkBlue" />
        <SolidColorBrush x:Key="PivotHeaderForegroundUnselectedBrush" Color="Gray" />

        <local:ThumbnailIconLinkGenerator x:Key="ThumbnailIconLinkGenerator"/>
        <local:FileSizeConverter x:Key="FileSizeConverter"/>
        <local:PermissionRoleTypeConverter x:Key="PermissionRoleTypeConverter"/>
        <local:PhoneAccentConverter x:Key="PhoneAccentConverter"/>

        <DataTemplate x:Key="MultiSelectItemDataTemplate">
            <Grid Height="120" Name="FilesGrid">
                <Image Name="FileImg" Height="115"
                       Width="115" HorizontalAlignment="Left" Margin="2,0,0,0"
                               Source="{Binding Converter={StaticResource ThumbnailIconLinkGenerator}}"
                               Stretch="UniformToFill" />
                <TextBlock Text="{Binding Title}" Margin="125,10,0,0" Width="250" Height="30"
                         HorizontalAlignment="Left" FontSize="15" Foreground="Black" VerticalAlignment="Top" />
                <TextBlock Text="{Binding ModifiedDate}"
                                   Foreground="Gray" FontSize="12" Width="200" Height="20" Margin="130,10,0,0" HorizontalAlignment="Left" VerticalAlignment="Center"></TextBlock>
                <TextBlock Text="{Binding Path=FileSize, Converter={StaticResource FileSizeConverter}}"
                                   Foreground="Gray" FontSize="12" Width="200" Height="20" Margin="130,50,0,0" HorizontalAlignment="Left" VerticalAlignment="Center"></TextBlock>
            </Grid>
        </DataTemplate>
    </Application.Resources>
</Application>