All layers in the design tool (except for text) can be set with multiple stroke parameters at the properties panel on the right side.
It enables to set stroke color, stroke color transparency, stroke width, and stroke position (center, inside, or outside).
Stroke position refers to the stroke that appears in the middle, inside, or outside of the path of the layer that is formed.
If the layer is a closed shape, the inside stroke will be internal the path, the outside stroke will be external the path, and the center stroke will be right on the path.
If the layer is an unclosed shape, only the center stroke will be used as there is no difference between inside and outside the line.
(In the following image, the solid blue line is the path of the rectangle, and the solid red line is the stroke of the path)
Click to know more about the gradient color settings for the stroke (similar basic principles as filling).