<h1 class="cp_h1title">見出しテキスト</h1>

.cp_h1title {
  position: relative;
  text-align: center;
  z-index: 1;
}
.cp_h1title::before,
.cp_h1title::after {
  position: absolute;
  content: '';
  display: block;
  z-index: -1;
}
.cp_h1title::before {
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: #a6ced3;
  clip-path: polygon(12px 0%, calc(100% - 12px) 0%, 100% 12px, 100% calc(100% - 12px), calc(100% - 12px) 100%, 12px 100%, 0% calc(100% - 12px), 0% 12px);
}
.cp_h1title::after {
  top: 3px;
  left: 3px;
  border: 1px solid #567553;
  width: calc(100% - 6px);
  height: calc(100% - 6px);
}