#181621

CSS

:root {
  --back-in-black: #181621;
  --back-in-black-rgb: 24 22 33;
  --back-in-black-oklch: oklch(0.208 0.022 292.7);
}

.element {
  color: var(--back-in-black);
  background-color: hsl(251 20% 11%);
  border-color: oklch(0.208 0.022 292.7);
}

Tailwind CSS v4

@theme {
  --color-back-in-black: #181621;
  --color-back-in-black-50: #f5f4fb;
  --color-back-in-black-100: #e8e7ef;
  --color-back-in-black-200: #d4d3dd;
  --color-back-in-black-300: #b8b5c2;
  --color-back-in-black-400: #9e9cab;
  --color-back-in-black-500: #868494;
  --color-back-in-black-600: #72707d;
  --color-back-in-black-700: #5b5964;
  --color-back-in-black-800: #42414a;
  --color-back-in-black-900: #2e2d33;
  --color-back-in-black-950: #1a1a1f;
}

SCSS

$back-in-black: #181621;
$back-in-black-rgb: 24, 22, 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);
}