Large or unusual tables can be hard to read. Adding a gray background on alternate rows can help mitigate this issue. Implementations of this feature can be found in HDTBL or can be crafted with a custom macro.
The HDTBL macro package provides many options and two of them are of interest here, fgc and bgc. As specified by the groff_hdtbl(7) man page these options control the background and foreground colour of the table cells.
The background colour of the table cells. This includes the area specified with the ‘csp’ argument. The argument ‘bgc=’ (no value) suppresses a background colour; this makes the background transparent.
Default: ‘bgc=bisque’ (string ‘t*bgc’).
The foreground colour of the cell contents.
Default: ‘fgc=red4’ (string ‘t*fgc’).
One can also use the tbl(1) preprocessor with a custom macro. Tadziu Hoffmann did just that and shared it (2021-11/msg00110).
Blake McBride used this macro and encountered some issues when a table spans pages so he edited the macro and also shared it (2021-11/msg00103).