Howdy, @nasiralamreeki!
Thanks for writing in! The section IDs on our live install will be different from your local based on the page ID, but I assume you’re talking about this border here:
If that is the case, it is a bottom border or the white Section above. Inspect that Section (and make sure you’re on the Section Element, not its child Rows or Columns) and then go to the border control, then make sure the bottom border button is selected:
If the border control is “linked,” you will not see that side selection, so make sure you unlink the sides, then you can select the bottom border, and from there you will be able to adjust the color or width or style independently of the other sides.
Hopefully that helps to point you in the right direction, cheers!