May b I am wrong, but imo i think the way script is written for the left sidebar isn’t the best way. If you take a close look, you can see that whenever you click the heading, the element next to it starts to increase the size(going from display none to block via js). The good way is to use some kinda of accordian plugin.
Now for the css part, give me some time, i’ll paste the code here which will match the reference site.
P.S: it would be a good idea to make a demo site/link where you can test the code and implement it to the main site.