Why?

css:

.mainMenuHeader {color:#FFFFFF;}
.mainMenuHeader:hover {color:#F26540;}

xaml:

<Style x:Key="mainMenuHeader">
  <Setter Property="Control.Foreground" Value="#FFFFFF" />
  <Style.Triggers>
    <Trigger Property="Control.IsMouseOver" Value="True">
      <Setter Property="Control.Foreground" Value="#F26540" />
    </Trigger>
  </Style.Triggers>
</Style>