<div class="wrap">
<div class="head">上の要素</div>
  <div class="main">
    <div class="main_inner">斜め背景の要素<br>内側に要素を追加し、上下の直角三角形の高さ分だけ上下にマージンをとる</div>
  </div>
<div class="foot">下の要素</div>
</div>

.wrap {
  overflow: hidden;
}
.head {
  background: gray;
  padding: 50px 0;
  text-align: center;
}

.foot{
  background: gray;
    padding: 50px 0;
  text-align: center;
}

.main {
  text-align:center;
  color: white;
  margin: calc(100vw * 0.08748866352 / 2) 0;
  position: relative;
  padding: 1px 0;

  &:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
    background: black;
 transform: skewY(5deg);
    z-index: -1;
  }
}
.main_inner {
  margin: calc(100vw * 0.08748866352 / 2) 0;
      border-top: 1px solid red;
  border-bottom: 1px solid red;


}