The reason that was not working was that I forgot to compile from LESS to CSS.
Every time somebody is making changes to LESS files, then those files must be compiled to CSS (see attached picture in order to find that function).
The problem is that despite the fact the width of the off-canvas can be changed, it is not overlapping the main website. So, the width change only the module inside the off-canvas. It can be reduced (for the module) but the width will remain the same (250px).