Border width, color, style or a brdr() object. See borders.
For padding, padding width in points.
Details
set_outer_borders sets borders round the top, bottom, left and
right of a group of cells. Behaviour is undefined unless row and col
specify contiguous sequences. set_outer_border_colors and
set_outer_border_styles set border colors and styles. set_outer_padding
sets padding, i.e. top padding on the top row of cells, etc.