How to only have thick border bellow a line of cells

[conualfy]

I am trying to format a table with a school schedule, but I am definitely having problems. I simply cannot set a different kind of cell borders as I would like to. On the left, I have a column with weekdays and each has multiple lines with times and subjects. Between days I would like to have a thicker border as I already did in most, but now as I edited the file, I broke some and I am not able to set this kind of border between days without setting a thick border on all sides of the edited cells.
I tried both the controls in the toolbar and Cell properties-Borders, but they seem to work different and I cannot make my thing with it.

I attached the file, I hope someone can teach me how to control those borders. Thank you very much.

orar-anul-1-2-semestrul-2.ods

[Hrbrgr]

Select cells in column A.

From the menu, choose Format>Cells. In the Format Cells dialog box, click the top and bottom lines [1] until they are light gray. Now click the right line [2], it must be black. At “Width”, select the line width and OK.

Sorry for my German-UI. I hope it will help you anyway. I wish you success!

[conualfy]

So, first making gray those I wish to stay as they are, then select (black) and edit the one I want to change. I did that, but I get different results than cells on the right and left which have a border set from the toolbox with the border style no. 4 (looks like a double line, the first double line). Then when I tried to apply that double line I got the line on all sides, although in the toolbar I set it only for bottom line (see C16, “Arta în spațiul României în Evul Mediu și Renaștere
(Ș. Solcan) Amf. Condurachi”).