#171721

CSS

:root {
  --back-in-black: #171721;
  --back-in-black-rgb: 23 23 33;
  --back-in-black-oklch: oklch(0.209 0.02 284.6);
}

.element {
  color: var(--back-in-black);
  background-color: hsl(240 18% 11%);
  border-color: oklch(0.209 0.02 284.6);
}

Tailwind CSS v4

@theme {
  --color-back-in-black: #171721;
  --color-back-in-black-50: #f4f4fb;
  --color-back-in-black-100: #e7e7ee;
  --color-back-in-black-200: #d3d3dc;
  --color-back-in-black-300: #b6b6c1;
  --color-back-in-black-400: #9d9daa;
  --color-back-in-black-500: #848593;
  --color-back-in-black-600: #70707c;
  --color-back-in-black-700: #595a64;
  --color-back-in-black-800: #414249;
  --color-back-in-black-900: #2d2d33;
  --color-back-in-black-950: #1a1a1e;
}

SCSS

$back-in-black: #171721;
$back-in-black-rgb: 23, 23, 33;
$back-in-black-light: mix(#fff, $back-in-black, 20%);
$back-in-black-dark: mix(#000, $back-in-black, 20%);

.element {
  color: $back-in-black;
  box-shadow: 0 0 0 2px rgba($back-in-black, 0.35);
}