"Cannot locate Resource "themes.xxx.xaml"" runtime error - WPF Custom Components in Another Assembly

I am trying to use custom controls that I created in one class library assembly in my WPF project. However, I keep getting the runtime error "Cannot locate resource 'themes/rcttextbox.xaml'" when I launch my application.

In my custom control library, "SoftwareThemeDesigner", I have the following Generic.xaml code in my "Themes" folder:

<!-- Original Generic.xaml file in "Themes" folder -->
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="/Themes/RctTextBox.xaml" />
    </ResourceDictionary.MergedDictionaries>

</ResourceDictionary>

The /Themes/RctTextBox.xaml is a resource dictionary with a <Style> that has a ContentTemplate, as per the default "Custom Control" file when creating one in Visual Studio.

In my "SoftwareThemeDesignTester" WPF application that references my "SoftwareThemeDesigner" library, in the App.xaml file, I have the following code:

<!-- WPF App's App.xaml file -->
<Application x:Class="SoftwareThemeDesignerTester.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/SoftwareThemeDesigner;component/Themes/Generic.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

</Application>

What I've tried:

1. Changing all the build actions of every resource dictionary from "Page" to "Resource"
2. Adding the /SoftwareThemeDesinger;component/ prefix to each resource dictionary in the Generic.xaml file
3. Moved the RctTextBox.xaml code to Generic.xaml and removed both the resource dictionary file and the reference to it (See Below).

<!-- Modified Generic.xaml file after removing references to other resource dictionaries -->
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Style TargetType="{x:Type local:RctTextBox}"
           BasedOn="{StaticResource {x:Type TextBox}}>
        <!-- RctTextBox Style details placed here -->
    </Style>


</ResourceDictionary>

This has worked, but it's strange that I can't use a separate resource dictionary, and it may not be practical to have to define everything in the generic.xaml file in the "Themes" folder.

Can any of you please advise? I've spend hours googling a solution and none have worked. I feel like I'm missing something.

Thanks in advance!

Updated Code based on @Bizhan's answer

<!-- Generic.xaml file in "Themes" folder -->
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="pack://application:,,,/SoftwareThemeDesigner;component/Themes/RctTextBox.xaml" />
    </ResourceDictionary.MergedDictionaries>

</ResourceDictionary>

<!-- WPF App's App.xaml file -->
<Application x:Class="SoftwareThemeDesignerTester.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="pack://application:,,,/SoftwareThemeDesigner;component/Themes/Generic.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

</Application>