Add project files.

ViewModels/Pages/SettingsViewModel.cs

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+// This Source Code Form is subject to the terms of the MIT License.
+// If a copy of the MIT was not distributed with this file, You can obtain one at https://opensource.org/licenses/MIT.
+// Copyright (C) Leszek Pomianowski and WPF UI Contributors.
+// All Rights Reserved.
+
+using Wpf.Ui.Appearance;
+using Wpf.Ui.Controls;
+
+namespace DownloadManager.ViewModels.Pages;
+public partial class SettingsViewModel : ObservableObject, INavigationAware
+{
+    private bool _isInitialized = false;
+
+    [ObservableProperty]
+    private string _appVersion = String.Empty;
+
+    [ObservableProperty]
+    private ApplicationTheme _currentTheme = ApplicationTheme.Unknown;
+
+    public void OnNavigatedTo()
+    {
+        if (!_isInitialized)
+            InitializeViewModel();
+    }
+
+    public void OnNavigatedFrom()
+    {
+    }
+
+    private void InitializeViewModel()
+    {
+        CurrentTheme = ApplicationThemeManager.GetAppTheme();
+        AppVersion = $"UiDesktopApp1 - {GetAssemblyVersion()}";
+
+        _isInitialized = true;
+    }
+
+    private string GetAssemblyVersion()
+    {
+        return System.Reflection.Assembly.GetExecutingAssembly().GetName().Version?.ToString()
+            ?? String.Empty;
+    }
+
+    [RelayCommand]
+    private void OnChangeTheme(string parameter)
+    {
+        switch (parameter)
+        {
+            case "theme_light":
+                if (CurrentTheme == ApplicationTheme.Light)
+                    break;
+
+                ApplicationThemeManager.Apply(ApplicationTheme.Light);
+                CurrentTheme = ApplicationTheme.Light;
+
+                break;
+
+            default:
+                if (CurrentTheme == ApplicationTheme.Dark)
+                    break;
+
+                ApplicationThemeManager.Apply(ApplicationTheme.Dark);
+                CurrentTheme = ApplicationTheme.Dark;
+
+                break;
+        }
+    }
+}