You'll normally want your tooltips fully opaque so that the user can easily read the content on the tooltip. However, there might be cases when you'd like the tooltip at least partially transparent, to hint at content behind the tooltip.
To make a tooltip partially transparent, you can set its Background property to a color that includes an alpha value indicating transparency.
<Button Content="Read a Book"
<TextBlock Text="Reading Suggestion" FontWeight="Bold"/>
<TextBlock Text="Charles Dickens" Margin="5"/>
Read more: 2,000 Things You Should Know About WPF