"The lines forming the columns in the spreadsheet are not black by default."? - When I am in Excel and check the color of the lines that make the grid, they are a brownish color. (I will email you the screen shot - I don't see a way to post here.)
"Also, can you please tell me the report that you are running and the style sheet that you are using?" - It's a custom style sheet that Bentley did for us. (I will email that also.) The reports that we use are custom ones created by Bentley for us. This happens with all of our reports.
We noticed that not all of the lines would always print out on the hard copy/PDF. But they are there in the DGN. Someone happen to notice that the line color wasn't black in Excel. When they switch it to black, then it prints fine.
One user did make some change in a theme by editing the "theme.css" file. It fixed the issue with the black lines, but now his word/excel won't open. So we do not want to try that with anyone else yet.