href-plus/source/scss/themes/_high-contrast.scss

.high-contrast-black {
  --foreground-1: #fff;
  --foreground-2: #ddd;
  --background-1: #000;
  --background-2: #222;
}

.high-contrast-white {
  --foreground-1: #000;
  --foreground-2: #222;
  --background-1: #fff;
  --background-2: #ddd;
}