User:Star0/sandbox/styles.css

.CCsingle {
  position: relative;
}

.CCsingle::after {
  content: '';
  position: absolute;
  top: 0;
  right: 0;
  width: 50%;
  height: 50%;
  background-color: #9a337a;
  clip-path: polygon(100% 0, 0 0, 100% 100%);
}