rezise contour line

When I draw an e.g. rectangle with a contour line size of zero and filled the area with any color but afterwards (having saved and reopened the file) would like to increase the thickness and give that contour line a different color than the area I fail to do so. If, however, originally the line thickness has been greater than zero it is no problem. Does anybody has an idea how to change a contour line of zero later?

Select the drawing in the Drawing window. Right-click and Convert Strokes to Pencil Lines and you are good to go by choosing a line preset or adjusting line thickness manually.

Thanks!! That was exactly what I was looking for.