方案一

body {
  -webkit-filter: grayscale(100%);
  -moz-filter: grayscale(100%);
  -ms-filter: grayscale(100%);
  -o-filter: grayscale(100%);
  filter: grayscale(100%);
}

方案二

html {
      background: #ffffff;
      position: relative;
}
html::before {
      content: "";
      position: absolute;
      inset: 0;
      pointer-events: none;
      z-index: 999;
      mix-blend-mode: color;
      background: rgba(0, 0, 0, 0.5);
}