Seperating categories

Hello again,
I’m still experimenting with the three column design and have stumbled into yet another problem, I would like seperating lines between the different categories on the left hand side (Categories, other, meta etc.) .
Getting this to work on the right hand side went without any problems, but, as I am new to CSS, I still haven’t quite grasped it all yet.
You can find the current template here:
https://www.vandenarend.com/marcel/index.php