Opacity Property
Opacity denotes the appearance of a gradient in terms of its interaction with its background.
The default is 100; entirely opaque. Nothing behind the gradient will be visible at all.
As the value decreases, more and more of the background will become visible through the control. When the value is 0 the control will be entirely transparent and only the background will be visible.
When specified on a control, opacity affects the whole of the control, both foreground and background. When specified on a brush, opacity can be used to target either the foreground or the background, allowing for a semi-transparent background, but still fully opaque text.