Double line cell border (sum)

I'll sometimes use an 'Outside' Border on the row below then close the row up.
Unfortunately you are limited to a min .30cm height row so although you have a double line the lines are a bit far apart.
Numbers feedback under Numbers on the Menu bar is always a good idea.
S.

If there is, I haven't found it. Best I've found is a thick line.

Until Apple changes the code, you can use the Shapes graphics to add double lines between your list and sum.

Jerry