Container widget.
Then use the

BoxDecoration object, and use

The gradient “angle” is based on the Alignment (x, y) values:

If the beginning and ending x values are equal,
the gradient is vertical (0° | 180°).

If the beginning and ending y values are equal,
the gradient is horizontal (90° | 270°).

Vertical gradient

background: linear-gradient(180deg, #ef5350, rgba(0, 0, 0, 0) 80%);
}