对角线

background: linear-gradient(
    to top right,
    rgba(0, 0, 0, 0) 0%,
    rgba(0, 0, 0, 0) calc(50% - 1.5px),
    rgba(0, 0, 0, 1) 50%,
    rgba(0, 0, 0, 0) calc(50% + 1.5px),
    rgba(0, 0, 0, 0) 100%
  ), linear-gradient(
    to bottom right,
    rgba(0, 0, 0, 0) 0%,
    rgba(0, 0, 0, 0) calc(50% - 1.5px),
    rgba(0, 0, 0, 1) 50%,
    rgba(0, 0, 0, 0) calc(50% + 1.5px),
    rgba(0, 0, 0, 0) 100%
  );