Win10 is not a variant in the sense that Clean and Visual Studio are. There is no Styles/Win10/ folder and no dictionary to merge. What the library ships is a handful of individually keyed styles that you apply one control at a time.

<CheckBox Content="a check box" Style="{DynamicResource MahApps.Styles.CheckBox.Win10}" />
<RadioButton Content="a radio button" Style="{DynamicResource MahApps.Styles.RadioButton.Win10}" />
<Slider Value="40" Style="{DynamicResource MahApps.Styles.Slider.Win10}" />

The Win10 check box is a filled accent square rather than an outline with a tick, the radio button a ring with a solid centre, and the slider thumb a rounded pill instead of a bar.

What the library provides

Each brings its own supporting pieces — MahApps.Templates.Slider.Horizontal.Win10 and .Vertical.Win10, MahApps.Styles.Thumb.Slider.Win10, the two RepeatButton.Slider.*Track.Win10 styles, the range slider's thumb and templates, and light and dark close-button styles for the window buttons.

To apply one everywhere, declare an implicit style based on it:

<Style BasedOn="{StaticResource MahApps.Styles.CheckBox.Win10}" TargetType="{x:Type CheckBox}" />

Additions from this site

The library stops at those six. For several other controls this documentation ships drop-in dictionaries in the same look, written for these pages and not part of the package:

Download them, merge them after Controls.xaml, and read the page each one belongs to — several note what the built-in templates do and do not let a style reach.

Related

WinUI for the newer Fluent look, which the library does not cover at all. Clean and Visual Studio for the two real variants.