The Background can be any brush. It is not limited to being a single color. If you have your plaid in an image you can use an ImageBrush:
<ScrollViewer Height="300" Width="300"> <ScrollViewer.Background>
<ImageBrush ImageSource="/Images/plaid.png" Stretch="Fill">
</ImageBrush>
</ScrollViewer.Background>
</ScrollViewer>
That said, I suspect you don't really want the plaid on the background but want the alternating colors on elements within the ScrollViewer. For that you would need a collection control and a DataTemplate which alternated its Background based on index. If
you search on
Alternating Rows Silverlight you should find several posts where people discuss how to do this in Silverlight. The general idea will be the same.
--Rob