Thanks for the quick reply! Yes, I at first tried Cairo PNG, but then read about how it shouldn't be used, so I did an "export bitmap", and still had the problem, which was when I posted.
After reading your reply, I went back and took a look, and I think I found the problem. It's not with the exporting, but with setting the stroke colors. What happened was, I drew some closed shapes using the line-and-bezier pen, then used the fill
tool to set the fill colors. My mistake was thinking that the borders of the colored regions would be the lines and beziers I had drawn. Actually, the colored regions had their own strokes. Thus, even after setting the fill and stroke colors (or unsetting the stroke colors in some cases) of these colored regions/objects, the original thin lines were still there, and still black. I thought I had selected the original thin lines, but now I find that I need to zoom in quite a bit to select them, then I can delete them, change their color, etc.
Thanks for your help
. And in case anyone is puzzled but I said, attached is the svg file that shows the shapes and the original black lines. I hope this can help another beginner someday ..