calc column and row headers - grid lines and shading have disappeared

Using LibreOffice Calc on Ubuntu 17.10, there are no grid lines or shading on column or row headings. The letters and numbers appear, on a white background. All the other grid lines are fine - but they do not extend to the headings. Does anyone know how to get the grid lines and (usually gray) shading back?

If you ever find out an answer to this I’d love to hear about it. I’m trying to get kind of the opposite effect. I don’t mind the grid lines in the headers, but i really want to get rid of the two-tone shading.