WPF Resource Issue

<Application x:Class="TitanSTUDIO.App.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:TitanSTUDIO.App">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Themes/GenericTheme.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:custom="clr-namespace:TitanSTUDIO.App">

    <Style x:Key="HeaderTitle" TargetType="TextBlock">
        <Setter Property="Foreground" Value="#ffffff" />
        <Setter Property="FontSize" Value="16" />
        <Setter Property="FontWeight" Value="SemiBold" />
        <Setter Property="Margin" Value="40 30 0 15" />
    </Style>
</ResourceDictionary>