Setting Button FlatStyle in WPF
The ToolBar class defines a Style that makes Buttons look flat. An example of using it is:
<Button Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}"/>This also works for a ToggleButton when using ToggleButtonStyleKey.
WPF lets you completely restyle controls to make them look like whatever you want, which is why it doesn't have such a specific FlatStyle property on the Button control.