Numbers Gridlines

Hi,

First select format upper right corner

then choose “cell”

Select the cell(s) you want to modify and click which line you like to change. For example in my photo I choose the upper line.

after they you have lot of option. Line type, color, width

Enjoy!

Please read this

Change table gridlines and colors in Numbers on MacOfficial Apple Support › guide › numbers › mac

Ralf