<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Button MinWidth="75" Margin="10" FontSize="50">
  <Button.Style>
    <Style TargetType="{x:Type Button}">
    <Style.Triggers>
      <Trigger Property="IsMouseOver" Value="True">
        <Setter Property="Foreground" Value="Blue"/>
      </Trigger>
    </Style.Triggers>
    </Style>
  </Button.Style>
    OK
  </Button>
</Page>