Css Issue_ Looking for "where to look"

[hyggli]

Hello, these are my problems with vertical bar. I don't need to know all the code, I just need to know where to look

I tried to joke with theme.css but didn't find settings regarding this. Surely I'm nooby, although those should be little code mods

I'd need:

1.In the homepage for desktops to mantain sidebar locked(leftmost arrow) and keep it removable(rightmost arrow) on other devices

2.I need to find how to remove a piece of the vertical bar on research categories' page(when someone clicks on categories on the sidebar-menu ) (look down)

In this case, I should be able to line up the orange piece of the sidebar with the side menu

 

Thanks to whoever helps me ^^''

Regards

David