What I do to give the look of a border along one edge of a rectangle in ppt is set a background with a gradient of the color I want the border to be in, with first color 100% opacity at 0% location, and second color 0% opacity at 1% location.
If I want a wide border I set a third color. First is 100% opacity at 0% location, second is 100% opacity at location percentage that sets the width, and third is 0% opacity at +1% from there, so the "gradient" is imperceptible.
You can even add borders, of different weights if you want, on opposite edges of a rectangle by doing the same thing at the other end.
https://i.sstatic.net/xMbsp.png