getting application theme on app constructor makes theme not working properly

[kia-nasirzadeh]

Describe the bug

// comment following line and everything works, but having that line in application constructor make themes not working properly
var makebug = ApplicationThemeManager.GetAppTheme();

To Reproduce

to reproduce this just make a new wpf application (use .net-9 and wpf-ui 4.0.0-rc.3) and fill out following files like this:

---

App.xaml:

<Application x:Class="BugReportProject.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:BugReportProject"
             xmlns:ui="http://schemas.lepo.co/wpfui/2022/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ui:ThemesDictionary Theme="Dark" />
                <ui:ControlsDictionary />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

---

App.xaml.cs:

using System.Configuration;
using System.Data;
using System.Windows;
using Wpf.Ui.Appearance;

namespace BugReportProject
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        public App ()
        {
            // comment following line and everything works, but having that line in application constructor make themes not working properly
            var makebug = ApplicationThemeManager.GetAppTheme();
        }
    }

}

---

MainWindow.xaml:

<ui:FluentWindow
    x:Class="BugReportProject.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:BugReportProject"
    mc:Ignorable="d"
    Title="MainWindow" Height="450" Width="800"
    xmlns:ui="http://schemas.lepo.co/wpfui/2022/xaml">
    <StackPanel>
        <ui:TitleBar Title="WPF UI"/>
        <ui:Card Margin="8">
            <ui:Button Name="changeBtn" Content="change theme" Icon="{ui:SymbolIcon Fluent24}" />
        </ui:Card>
    </StackPanel>
</ui:FluentWindow>

---

MainWindow.xaml.cs:

using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using Wpf.Ui.Appearance;
using Wpf.Ui.Controls;

namespace BugReportProject
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : FluentWindow
    {
        public MainWindow()
        {
            InitializeComponent();
            changeBtn.Click += (s, e) =>
            {
                if (ApplicationThemeManager.GetAppTheme() == ApplicationTheme.Dark) ApplicationThemeManager.Apply(ApplicationTheme.Light);
                else ApplicationThemeManager.Apply(ApplicationTheme.Dark);
            };
        }
    }
}

Expected behavior

by clicking on changeBtn theme should switch between dark and light, but everything stucks if following line exists in App class constructor:
var makebug = ApplicationThemeManager.GetAppTheme();

Screenshots

No response

OS version

Microsoft windows
version 21H2 (OS Build 22000.3260)

.NET version

net9.0-windows

WPF-UI NuGet version

wpf-ui 4.0.0-rc.3

Additional context

No response

[kia-nasirzadeh]

this can be solved by getting app theme on startup:

public partial class App : Application
{
    public App ()
    {
        Startup += App_Startup;
    }

    private void App_Startup(object sender, StartupEventArgs e)
    {
        var x = ApplicationThemeManager.GetAppTheme();
    }
}