[tony_sologubov]

Originally Posted by Dr. Bogger

Can you point me in the right direction?
I don't think the media css will do what I need.

You can set up the CSS rule like this:

#sidebar-first {
float: none;
}

and it will achieve what you need.

Surely, if you want to add this rule depending on the device's width, the code would be something like this:

@media (max-width: 480px) {
  #sidebar-first {
    float: none;
  }
}