What exists instead is a set of drop-in dictionaries written for this documentation. They give individual controls a Fluent 2 look — rounded corners, the accent used sparingly, chrome that recedes — and they are not part of the NuGet package. Download the ones you want and merge them into your application.

The dictionaries

Merge them after the library's own Controls.xaml:

<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />
<ResourceDictionary Source="Styles/Controls.Calendar.WinUI.xaml" />
<ResourceDictionary Source="Styles/Controls.DateTimePicker.WinUI.xaml" />
<ResourceDictionary Source="Styles/Controls.ScrollBar.WinUI.xaml" />

Each defines keyed styles rather than implicit ones, so nothing changes until you apply them — either per control, or through an implicit style of your own:

<Style BasedOn="{StaticResource MahApps.Styles.Calendar.WinUI}" TargetType="{x:Type Calendar}" />

The DateTimePicker dictionary needs the Calendar one merged first, since its picker styles point at the WinUI calendar.

Styles, not replacement templates

Where it was possible, these dictionaries change only what the library's templates already expose — a corner radius through ControlsHelper.CornerRadius, a CalendarStyle, an IsClockVisible. That keeps them working when the library's templates change underneath.

Where it was not possible, the pages say so plainly rather than pretending. Two examples worth knowing before you start:

• the DateTimePicker's drop-down frame stays square whatever you do, because PART_PopupBorder has no CornerRadius and none of its brushes is a TemplateBinding — tracked as #4582
• the ScrollBar dictionary is a template replacement, because a WinUI scrollbar's expand-on-hover behaviour cannot be reached from a style at all

Related

Win 10 (UWP) — the library does ship a few control styles in that older look, and this site adds more. Clean and Visual Studio are the two variants that really are in the package.