Why can't I modify my "Double break margin" via .CSS?

I was told to modify the line-height in the code below but nothing happens when I change it.

}
.main-content .node .content, .comment .content, .book .body {
  line-height: 1.5em;

Basically I just want when I break twice there is a decent margin gap, but I don’t want to sacrifice the margin gab between the normal lines.

Currently the gap margin when I double break is too weak