LibreOffice Calc - bold or faint grid lines

I have been given an Excel document to edit. I have saved it as a .ods document. The original document had sections separated by bold grid lines and within each section there are subsections separated by faint lines. As I edit the document some of the bold lines are changing to faint. Is there a way that I can change a faint line back to bold or vice versa, please?

Thank you

The grid lines are just a ‘Visual Aid’. You only can set their color and choose per sheet if they shall be displayed or not. No width poperty! For the page format, tab ‘Sheet’, you can in addition choose if the grid lines shall be printed.
Another kind of visual aid are the differently looking lines marking the page wrap.

Any lines that can be changed in width (thickness) are borders. You can set and edit borders via the ‘Format Cells…’ dialog. A while ago there was an issue (bug) concerning the width of border lines. Some values / small differences were not correctly saved. With Versions 5.3 or higher the issue should be fixed.

If your actual problem was different, I need better information.

Thank you very much