Border under header only in json theme

[MadziaMal]

Hi all

i looking for how to make json file to import into power bi theme, and achive border only under header in table and matrix table

it need to be done by json theme file, im looking everywere, but i cant find specification of that file

it shoul look that (i can only do that by changegin visual option for each)

many thank for help how to figure that