When you press the “d” button it will select the eyedropper tool. If you’re selecting something then you can click something to get that color, but If you’re selecting a line it changes the fill color.
It would be cool if every time you use the eyedropper tool while selecting a line; It changes the color of the line instead of the fill.
An alternative is to just select the line and use the eyedropper in its Inspector, but this solution is much quicker.