EDIT: did not look carefully on your code... :) so for completeness
<Window.Resources>
<Style x:Key="MyDataGridStyle" TargetType="DataGridRow">
<Setter Property="FontSize" Value="12"/>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Blue"/>
<Setter Property="Foreground" Value="White"/>
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
<DataGrid Name="DataGrid1"
ItemsSource="{Binding Path=Fibers}"
RowStyle="{StaticResource MyDataGridStyle}" ...>