Line around right sidebar

Hello there,

I am new to these forums and also fairly new to wordpress, not a pro when it comes to css and that kind of stuff, just trying to put a nice page togther for my little, starting business.

The website is under https://www.stress-aktiv-begegnen.de

My question is: Is is possible to put a thin grey line/box around the sidebar on the right? I have managed to change the font in the sidebar, but have no idea how to do the rest.

Any help would be very much appreciated, thank you!