Estou tentando definir um estilo para o meu controle de usuário. O UserControl está em um projeto "Controles" e o tema está em um projeto "MainProject"

<UserControl x:Class="Controls.OutputPanel"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        mc:Ignorable="d" 
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        x:Name="OutputControl"> 
   <!-- Style="{DynamicResource UserControlStyle}"> - I cant set the style here because the Resource Dictionary hasn't been defined yet -->

    <UserControl.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/MainProject;component/Themes/MyTheme.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </UserControl.Resources>

    <!-- Now that the Resource Dictionary has been defined I need to set the style -->      

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>

        <TextBox x:Name="textbox" 
                   ScrollViewer.VerticalScrollBarVisibility="Visible"
                   Text="{Binding ElementName=OutputControl, Path=TextProperty}"
                   IsReadOnly="True"
                   Style="{DynamicResource OutputTextBoxStyle}"/>

    </Grid>

</UserControl>