The background and foreground of all elements except the chart itself can be “Transparent”.

When a background or foreground is transparent, the chart uses the color of the element outside it for the background. For example, the header would have the background of the chart itself when its background is set to Transparent.

In other words, if the background color of the element is transparent then its background is not drawn.  If the foreground color of the element is transparent, then the foreground (for example the text of a title) is not drawn. 

The transparent color options are on the Web tab of the .NET color dialog.