Row separator lines not showing

My client is complaining that now that we upgraded to the new version (2.5.5), the lines that delineated the rows have disappeared. I thought at first that this might be a function of a row height, but I cannot find any indication of such a property here.

Does anyone know how to bring back the row separators (horizontal lines separating the rows), or to make them more noticeable?

Thanks in advance!

Thanks! That did it!