Sidebar Box making

Hi there. I am curently a new user of WordPress and CSS.
On my site ,I have Rara Clean Theme installed. On my right side, the sidebar is not integrated in a box , as the one in the center,as I would like it to be.
Thing is that I don’t know where(in what file and where in it), and what to take, specifically to edit, copy and paste the code for the sidebar to be in a box.

Example of what I want to do

Thank you!