diff --git a/src/OrthoStreamXP/Gui/App.xaml.cs b/src/OrthoStreamXP/Gui/App.xaml.cs
index c4e43ba..858f194 100644
--- a/src/OrthoStreamXP/Gui/App.xaml.cs
+++ b/src/OrthoStreamXP/Gui/App.xaml.cs
@@ -25,6 +25,7 @@ protected override void OnStartup(StartupEventArgs e)
         Log.Configure(LogOutput.None, null);
         DispatcherUnhandledException += OnUnhandledException;
         _main = new MainViewModel();
+        _main.Preferences.Theme.Apply();
         var window = new MainWindow(_main);
         MainWindow = window;
         window.Show();
diff --git a/src/OrthoStreamXP/Gui/AppTheme.cs b/src/OrthoStreamXP/Gui/AppTheme.cs
new file mode 100644
index 0000000..42feb26
--- /dev/null
+++ b/src/OrthoStreamXP/Gui/AppTheme.cs
@@ -0,0 +1,71 @@
+using System.Windows;
+using Microsoft.Win32;
+
+namespace OrthoStreamXP.Gui;
+
+/// <summary>The window's colours: the Windows theme (follows Windows), or always light or dark.</summary>
+internal enum AppTheme
+{
+    System,
+    Light,
+    Dark,
+}
+
+/// <summary>The sidebar's theme button: what it shows for a theme, and the theme a click switches to.</summary>
+internal sealed record ThemeChoice(string Label, string Glyph, AppTheme Theme)
+{
+    public static ThemeChoice Windows { get; } = new("Windows theme", "", AppTheme.System);
+    public static ThemeChoice Light { get; } = new("Light theme", "", AppTheme.Light);
+    public static ThemeChoice Dark { get; } = new("Dark theme", "", AppTheme.Dark);
+
+    public static ThemeChoice Of(AppTheme theme) => theme switch
+    {
+        AppTheme.Light => Light,
+        AppTheme.Dark => Dark,
+        _ => Windows,
+    };
+
+    /// <summary>
+    /// The Windows theme (follows Windows), then the other look, then Windows' look kept fixed, then the Windows theme again:
+    /// the first click always changes the colours.
+    /// </summary>
+    public ThemeChoice Next(Func<bool> windowsUsesLight)
+    {
+        ThemeChoice same = windowsUsesLight() ? Light : Dark;
+        ThemeChoice other = same == Light ? Dark : Light;
+        if (this == Windows) return other;
+        return this == other ? same : Windows;
+    }
+}
+
+internal static class AppThemeExtensions
+{
+#pragma warning disable WPF0001 // WPF's Fluent theme is marked experimental.
+    /// <summary>The WPF Fluent theme mode for the choice.</summary>
+    public static ThemeMode ToThemeMode(this AppTheme theme) => theme switch
+    {
+        AppTheme.Light => ThemeMode.Light,
+        AppTheme.Dark => ThemeMode.Dark,
+        _ => ThemeMode.System,
+    };
+
+    /// <summary>Switches the running application to the theme; the window's DynamicResource brushes follow.</summary>
+    public static void Apply(this AppTheme theme)
+    {
+        if (Application.Current is { } app) app.ThemeMode = theme.ToThemeMode();
+    }
+#pragma warning restore WPF0001
+}
+
+/// <summary>Whether Windows shows apps light or dark (Settings > Personalisation > Colours).</summary>
+internal static class WindowsTheme
+{
+    private const string Personalize = @"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize";
+
+    /// <summary>True for light apps, also when Windows does not say (older versions default to light).</summary>
+    public static bool AppsUseLight()
+    {
+        using RegistryKey? key = Registry.CurrentUser.OpenSubKey(Personalize);
+        return key?.GetValue("AppsUseLightTheme") is not int value || value != 0;
+    }
+}
diff --git a/src/OrthoStreamXP/Gui/MainWindow.xaml b/src/OrthoStreamXP/Gui/MainWindow.xaml
index 419800c..d29c9fc 100644
--- a/src/OrthoStreamXP/Gui/MainWindow.xaml
+++ b/src/OrthoStreamXP/Gui/MainWindow.xaml
@@ -66,6 +66,16 @@
                     </StackPanel>
                 </Button>
 
+                <!-- Theme: Windows (follows Windows), then the other look, then Windows' look kept fixed, then Windows again -->
+                <Button DockPanel.Dock="Bottom" Style="{StaticResource SidebarLink}" Margin="4,0,4,0"
+                        Command="{Binding ChangeTheme}" AutomationProperties.Name="{Binding Theme.Label, StringFormat='{}{0} (click to change)'}"
+                        ToolTip="Switch the window's colours: same as Windows, light or dark.">
+                    <StackPanel Orientation="Horizontal">
+                        <TextBlock Style="{StaticResource Icon}" Text="{Binding Theme.Glyph}" Width="20" />
+                        <TextBlock Text="{Binding Theme.Label}" Margin="12,0,0,0" VerticalAlignment="Center" />
+                    </StackPanel>
+                </Button>
+
                 <ListBox ItemsSource="{Binding Navigation}" SelectedItem="{Binding SelectedNav}"
                          ItemContainerStyle="{StaticResource NavItem}" Background="Transparent" BorderThickness="0"
                          AutomationProperties.Name="Pages" ScrollViewer.HorizontalScrollBarVisibility="Disabled" />
diff --git a/src/OrthoStreamXP/Gui/Theme.xaml b/src/OrthoStreamXP/Gui/Theme.xaml
index 876aee5..ae7db94 100644
--- a/src/OrthoStreamXP/Gui/Theme.xaml
+++ b/src/OrthoStreamXP/Gui/Theme.xaml
@@ -8,6 +8,11 @@
     <gui:VisibleWhen x:Key="Visible" />
     <gui:EqualsParameter x:Key="Is" />
 
+    <!--
+      A Style set on a control replaces Fluent's implicit one, so the classic theme's black text would show through:
+      the styles below set the Foreground themselves.
+    -->
+
     <!-- Text -->
     <Style x:Key="PageTitle" TargetType="TextBlock">
         <Setter Property="FontSize" Value="28" />
@@ -116,6 +121,7 @@
 
     <!-- A sidebar link: looks like a navigation item, but opens a web page -->
     <Style x:Key="SidebarLink" TargetType="Button">
+        <Setter Property="Foreground" Value="{DynamicResource TextFillColorPrimaryBrush}" />
         <Setter Property="Margin" Value="4,2" />
         <Setter Property="Cursor" Value="Hand" />
         <Setter Property="FocusVisualStyle" Value="{DynamicResource DefaultControlFocusVisualStyle}" />
@@ -140,6 +146,7 @@
 
     <!-- A choice shown as a card (quality presets, detected folders): an accent border when chosen -->
     <Style x:Key="ChoiceCard" TargetType="RadioButton">
+        <Setter Property="Foreground" Value="{DynamicResource TextFillColorPrimaryBrush}" />
         <Setter Property="Cursor" Value="Hand" />
         <Setter Property="FocusVisualStyle" Value="{DynamicResource DefaultControlFocusVisualStyle}" />
         <Setter Property="Margin" Value="0,0,0,6" />
@@ -196,6 +203,7 @@
 
     <!-- An on/off setting as a Windows 11 toggle switch: its text ("On", "Off") on the left, the switch on the right -->
     <Style x:Key="Switch" TargetType="CheckBox">
+        <Setter Property="Foreground" Value="{DynamicResource TextFillColorPrimaryBrush}" />
         <Setter Property="Cursor" Value="Hand" />
         <Setter Property="MinWidth" Value="0" />
         <Setter Property="VerticalAlignment" Value="Center" />
diff --git a/src/OrthoStreamXP/Gui/ViewModels/MainViewModel.cs b/src/OrthoStreamXP/Gui/ViewModels/MainViewModel.cs
index 06d5873..8705524 100644
--- a/src/OrthoStreamXP/Gui/ViewModels/MainViewModel.cs
+++ b/src/OrthoStreamXP/Gui/ViewModels/MainViewModel.cs
@@ -1,4 +1,5 @@
 using System.Text.Json;
+using System.Text.Json.Serialization;
 using System.Windows.Input;
 using OrthoStreamXP.Setup;
 
@@ -25,6 +26,10 @@ internal sealed class WindowPreferences
     /// <summary>Download a mesh region's roads, forests and buildings (its overlays) with it.</summary>
     public bool DownloadOverlays { get; set; } = true;
 
+    /// <summary>Light, dark, or the same as Windows.</summary>
+    [JsonConverter(typeof(JsonStringEnumConverter<AppTheme>))]
+    public AppTheme Theme { get; set; }
+
     public static WindowPreferences Load()
     {
         try
@@ -79,6 +84,7 @@ public MainViewModel()
         _selected = Navigation[0];
         HandleProblem = new RelayCommand<ProblemAction>(Handle);
         Donate = new RelayCommand(() => Shell.Open(AboutViewModel.DonationPage));
+        ChangeTheme = new RelayCommand(() => Theme = Theme.Next(WindowsTheme.AppsUseLight));
         Mount.PropertyChanged += (_, e) =>
         {
             if (e.PropertyName == nameof(MountController.State)) Raise(nameof(SidebarStatus));
@@ -140,6 +146,23 @@ private set
     /// <summary>Opens the donation page, from the sidebar.</summary>
     public ICommand Donate { get; }
 
+    /// <summary>The sidebar's theme button: Windows, the other look than Windows', Windows' look kept fixed, back to Windows.</summary>
+    public ICommand ChangeTheme { get; }
+
+    /// <summary>The window's theme (the Windows one follows Windows): applied at once and remembered for the next start.</summary>
+    public ThemeChoice Theme
+    {
+        get => ThemeChoice.Of(Preferences.Theme);
+        private set
+        {
+            if (value.Theme == Preferences.Theme) return;
+            Preferences.Theme = value.Theme;
+            Preferences.Save();
+            value.Theme.Apply();
+            Raise();
+        }
+    }
+
     public void Show(IPage page) => SelectedNav = Navigation.First(n => n.Page == page);
 
     /// <summary>Called when the window has opened: setup on the first run, otherwise the auto-start.</summary>
diff --git a/tests/OrthoStreamXP.Tests/AppThemeTests.cs b/tests/OrthoStreamXP.Tests/AppThemeTests.cs
new file mode 100644
index 0000000..0571497
--- /dev/null
+++ b/tests/OrthoStreamXP.Tests/AppThemeTests.cs
@@ -0,0 +1,51 @@
+using System.Text.Json;
+using System.Windows;
+using OrthoStreamXP.Gui;
+
+namespace OrthoStreamXP.Tests;
+
+/// <summary>The window's theme choice: what each entry switches WPF to, and how window.json keeps it.</summary>
+public sealed class AppThemeTests
+{
+#pragma warning disable WPF0001
+    [Fact]
+    public void Each_choice_maps_to_its_wpf_theme_mode()
+    {
+        Assert.Equal(ThemeMode.System, AppTheme.System.ToThemeMode());
+        Assert.Equal(ThemeMode.Light, AppTheme.Light.ToThemeMode());
+        Assert.Equal(ThemeMode.Dark, AppTheme.Dark.ToThemeMode());
+    }
+#pragma warning restore WPF0001
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void Clicks_go_windows_then_the_other_look_then_the_windows_look_then_windows(bool windowsUsesLight)
+    {
+        AppTheme same = windowsUsesLight ? AppTheme.Light : AppTheme.Dark;
+        AppTheme other = windowsUsesLight ? AppTheme.Dark : AppTheme.Light;
+        ThemeChoice choice = ThemeChoice.Of(AppTheme.System);
+        var seen = new List<AppTheme>();
+        for (int i = 0; i < 4; i++, choice = choice.Next(() => windowsUsesLight)) seen.Add(choice.Theme);
+
+        Assert.Equal([AppTheme.System, other, same, AppTheme.System], seen);
+    }
+
+    [Fact]
+    public void Each_saved_theme_shows_as_its_own_choice()
+    {
+        Assert.Same(ThemeChoice.Windows, ThemeChoice.Of(AppTheme.System));
+        Assert.Same(ThemeChoice.Light, ThemeChoice.Of(AppTheme.Light));
+        Assert.Same(ThemeChoice.Dark, ThemeChoice.Of(AppTheme.Dark));
+    }
+
+    [Fact]
+    public void The_theme_is_kept_by_name_and_older_files_without_it_follow_windows()
+    {
+        string json = JsonSerializer.Serialize(new WindowPreferences { Theme = AppTheme.Dark });
+
+        Assert.Contains("\"Theme\":\"Dark\"", json, StringComparison.Ordinal);
+        Assert.Equal(AppTheme.Dark, JsonSerializer.Deserialize<WindowPreferences>(json)!.Theme);
+        Assert.Equal(AppTheme.System, JsonSerializer.Deserialize<WindowPreferences>("{\"AutoStart\":true}")!.Theme);
+    }
+}