ThemeManager is the class behind everything a theme does after startup: switching, detecting, following the Windows setting, and building themes that are not in the box. It lives in ControlzEx, not in MahApps.Metro, so every snippet here starts with

using ControlzEx.Theming;

and works through the singleton ThemeManager.Current. For the forty-six themes that ship with the library and how to pick one in App.xaml, see Usage.

Changing the theme

ThemeManager.Current.ChangeTheme(Application.Current, "Dark.Green");

That swaps the theme for the whole application, and every control follows immediately — the styles reach their colours through DynamicResource, so nothing needs recreating.

The same call takes a FrameworkElement instead, which is how one window, or one panel, gets a theme of its own while the rest of the application keeps the application theme:

public partial class MainWindow : MetroWindow
{
    public MainWindow()
    {
        this.InitializeComponent();

        ThemeManager.Current.ChangeTheme(this, "Dark.Red");
    }
}

In XAML the equivalent is merging the theme dictionary into the window's own resources:

<mah:MetroWindow.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Themes/Dark.Red.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</mah:MetroWindow.Resources>

One axis at a time

Two shortcuts change the base theme or the colour scheme and leave the other alone, which is what a light/dark toggle wants:

ThemeManager.Current.ChangeThemeBaseColor(Application.Current, "Dark");
ThemeManager.Current.ChangeThemeColorScheme(Application.Current, "Emerald");

Finding out what is applied

var current = ThemeManager.Current.DetectTheme(Application.Current);

if (current is not null)
{
    ThemeManager.Current.ChangeTheme(Application.Current, ThemeManager.Current.GetInverseTheme(current));
}

Themes is what to bind a theme picker to; each Theme carries a DisplayName, a BaseColorScheme, a ColorScheme and a ShowcaseBrush for the swatch.

Following Windows

ThemeSyncMode decides how much of the Windows personalisation setting the application adopts. It is a flags enum:

Set the mode, then sync once; after that the manager keeps up with changes on its own:

protected override void OnStartup(StartupEventArgs e)
{
    base.OnStartup(e);

    ThemeManager.Current.ThemeSyncMode = ThemeSyncMode.SyncAll;
    ThemeManager.Current.SyncTheme();
}

SyncWithAccent generates a theme from whatever colour the user picked, which is the mechanism the next section uses directly. On develop such a theme also takes the shades of the accent from Windows itself, so the WinUI controls come out in the colours a Windows 11 app has.

A theme from any colour

RuntimeThemeGenerator builds a complete theme — every brush, both the accent ramp and the greys — from a base theme and one colour. No dictionary to write:

var theme = RuntimeThemeGenerator.Current.GenerateRuntimeTheme("Light", Color.FromRgb(0x6A, 0x1B, 0x9A));

ThemeManager.Current.AddTheme(theme);
ThemeManager.Current.ChangeTheme(Application.Current, theme);

Each panel in the figure is that call with a different colour, applied to the panel rather than to the application — ChangeTheme takes an element, so a theme can be scoped to as little as one Border.

Generate both bases if the application has a light/dark toggle, so GetInverseTheme has somewhere to go:

foreach (var baseTheme in new[] { "Light", "Dark" })
{
    ThemeManager.Current.AddTheme(RuntimeThemeGenerator.Current.GenerateRuntimeTheme(baseTheme, brandColour));
}

AddTheme also takes a hand-built Theme, if you want to control the name and the showcase brush:

ThemeManager.Current.AddTheme(new Theme("CustomDarkRed", "CustomDarkRed", "Dark", "Red", Colors.DarkRed, Brushes.DarkRed, true, false));

A theme written by hand

When the generated ramp is not what you want — a brand palette with its own greys, say — write the dictionary yourself and register it as a library theme:

var theme = ThemeManager.Current.AddLibraryTheme(
    new LibraryTheme(
        new Uri("pack://application:,,,/SampleApp;component/CustomAccents/Light.Accent1.xaml"),
        MahAppsLibraryThemeProvider.DefaultInstance));

ThemeManager.Current.ChangeTheme(this, theme);

The dictionary needs seven metadata keys at the top, and the manager reads them to place the theme:

<system:String x:Key="Theme.Name">Light.Accent1</system:String>
<system:String x:Key="Theme.Origin">MahAppsMetroThemesSample</system:String>
<system:String x:Key="Theme.DisplayName">Accent1 (Light)</system:String>
<system:String x:Key="Theme.BaseColorScheme">Light</system:String>
<system:String x:Key="Theme.ColorScheme">Accent1</system:String>
<Color x:Key="Theme.PrimaryAccentColor">#FFD80073</Color>
<SolidColorBrush x:Key="Theme.ShowcaseBrush" Color="#FFD80073" options:Freeze="True" />

Everything after that is colours and brushes. A complete worked example is available as a file — five hundred lines, which is why it is not printed here.

A complete sample project is on GitHub.

Related

Usage for the themes that ship with the library and the naming, and the quick start for putting the first one in place.