#18171F

CSS

:root {
  --back-in-black: #18171f;
  --back-in-black-rgb: 24 23 31;
  --back-in-black-oklch: oklch(0.209 0.016 290.7);
}

.element {
  color: var(--back-in-black);
  background-color: hsl(248 15% 11%);
  border-color: oklch(0.209 0.016 290.7);
}

Tailwind CSS v4

@theme {
  --color-back-in-black: #18171f;
  --color-back-in-black-50: #f5f4f9;
  --color-back-in-black-100: #e8e7ed;
  --color-back-in-black-200: #d4d3da;
  --color-back-in-black-300: #b7b6bf;
  --color-back-in-black-400: #9e9da7;
  --color-back-in-black-500: #868590;
  --color-back-in-black-600: #71707a;
  --color-back-in-black-700: #5b5a62;
  --color-back-in-black-800: #424248;
  --color-back-in-black-900: #2d2d32;
  --color-back-in-black-950: #1a1a1e;
}

SCSS

$back-in-black: #18171f;
$back-in-black-rgb: 24, 23, 31;
$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);
}