fix(settings): apply a theme as soon as it is selected and restore it on cancel

PLAN 5.6 asks for the theme to take effect immediately, but it was applied
only on OK. Selecting a theme now previews it; OK saves it, and cancel, Escape
or closing the window restores the theme the dialog opened with.

YKanBan.Tests/ViewModels/SettingsDialogViewModelTests.cs

@@ -0,0 +1,63 @@
+using YKanBan.Storage.AppData;
+using YKanBan.ViewModels.Dialogs;
+
+namespace YKanBan.Tests.ViewModels;
+
+/// <summary>
+/// Tests for the settings dialog: the theme is previewed as soon as it is
+/// selected, saved on accept and restored on cancel.
+/// </summary>
+[TestClass]
+public class SettingsDialogViewModelTests
+{
+    [TestMethod]
+    public void OpeningTheDialogAppliesNothing()
+    {
+        var applied = new List<ThemeOption>();
+
+        _ = new SettingsDialogViewModel(new AppConfig(), AppConfig.DefaultLanguage, applied.Add);
+
+        Assert.AreEqual(0, applied.Count);
+    }
+
+    [TestMethod]
+    public void SelectingAThemeAppliesItImmediatelyWithoutSavingIt()
+    {
+        var config = new AppConfig();
+        var applied = new List<ThemeOption>();
+        var dialog = new SettingsDialogViewModel(config, AppConfig.DefaultLanguage, applied.Add);
+
+        dialog.SelectedTheme = dialog.Themes.Single(option => option.Value.Equals(ThemeOption.Dark));
+
+        CollectionAssert.AreEqual(new[] { ThemeOption.Dark }, applied);
+        Assert.AreEqual(ThemeOption.FollowSystem, config.Theme);
+    }
+
+    [TestMethod]
+    public void AcceptSavesThePreviewedTheme()
+    {
+        var config = new AppConfig();
+        var applied = new List<ThemeOption>();
+        var dialog = new SettingsDialogViewModel(config, AppConfig.DefaultLanguage, applied.Add);
+        dialog.SelectedTheme = dialog.Themes.Single(option => option.Value.Equals(ThemeOption.Light));
+
+        dialog.Accept();
+
+        Assert.AreEqual(ThemeOption.Light, config.Theme);
+        CollectionAssert.AreEqual(new[] { ThemeOption.Light }, applied);
+    }
+
+    [TestMethod]
+    public void CancelRestoresTheThemeTheDialogOpenedWith()
+    {
+        var config = new AppConfig { Theme = ThemeOption.Light };
+        var applied = new List<ThemeOption>();
+        var dialog = new SettingsDialogViewModel(config, AppConfig.DefaultLanguage, applied.Add);
+        dialog.SelectedTheme = dialog.Themes.Single(option => option.Value.Equals(ThemeOption.Dark));
+
+        dialog.Cancel();
+
+        Assert.AreEqual(ThemeOption.Light, config.Theme);
+        CollectionAssert.AreEqual(new[] { ThemeOption.Dark, ThemeOption.Light }, applied);
+    }
+}