The Clean variant replaces the accent-coloured window chrome with a white title bar, a centred title and dark buttons. It is the smaller of the library's two variants: it restyles the window and four other things, and leaves every ordinary control alone.

Opting in

Merge the variant's dictionary and set the window's style:

<mah:MetroWindow x:Class="MyApp.MainWindow"
                 xmlns:mah="http://metro.mahapps.com/winfx/xaml/controls"
                 Style="{DynamicResource MahApps.Styles.MetroWindow.Clean}">
    <mah:MetroWindow.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Clean/Controls.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </mah:MetroWindow.Resources>
</mah:MetroWindow>

That is what the demo's CleanWindowDemo does. Clean/Controls.xaml merges the five dictionaries below and declares implicit styles for each of them, so the explicit Style= on the window is belt-and-braces rather than required — but harmless, and clearer about intent.

Merge the dictionary into App.xaml instead if the whole application should use it.

What it restyles

The keyed styles are MahApps.Styles.MetroWindow.Clean, …WindowButtonCommands.Clean, …WindowCommands.Clean, …GroupBox.Clean, …StatusBar.Clean, plus MahApps.Styles.Separator.Clean and four Button.MetroWindow.*.Clean styles for the title-bar buttons in their light and dark forms.

Notice what is not in that list: buttons, text boxes, check boxes, lists, tabs. Clean is a window-chrome variant. If you want it to change more, it is a starting point to extend rather than a full theme.

Related

Visual Studio is the library's other variant and a much broader one. Win10 and WinUI are not variants in the same sense — see those pages.