<h1 class="cp_h1title">見出しテキスト</h1>

.cp_h1title {
	position: relative;
	background-color: #da3c3c;
	color: #fff;
	overflow: hidden;
	padding: .3em;
}
.cp_h1title:before{
	background-color: rgba(27, 37, 56, 1);
	content: '';
	display: block;
	transform: rotate(-30deg);
	position: absolute;
	bottom: -30px;
	right: -100px;
	width: 200px;
	height: 200px;
}