UnderlinedOnMouseOverButtonStyle.xaml

<Style x:Key="UnderlinedOnMouseOverButtonStyle" TargetType="{x:Type Button}">
	<Style.Triggers>
		<Trigger Property="IsMouseOver" Value="True">
			<Setter Property="ContentTemplate">
				<Setter.Value>
					<DataTemplate>
						<TextBlock TextDecorations="Underline" Text="{TemplateBinding Content}"/>
					</DataTemplate>
				</Setter.Value>
			</Setter>
		</Trigger>
		<Trigger Property="IsMouseOver" Value="False">
			<Setter Property="ContentTemplate">
				<Setter.Value>
					<DataTemplate>
						<TextBlock Text="{TemplateBinding Content}"/>
					</DataTemplate>
				</Setter.Value>
			</Setter>
		</Trigger>
	</Style.Triggers>
</Style>