Nothing happens when decreasing from 1em to 0em because the text above and below the list is inserted in p tags by wpautop function, and in my stylesheet I had set p { margin: 1em 0; } hence there is a 1em space which overlapps.
The problem is solved globally using the + adjacent sibling selector (read more here)
div + ul { margin-top: 0.5em; }
ul + div { margin-top: -0.5em; }
p + ul { margin-top: -0.5em; }
ul + p { margin-top: -0.5em; }