#191621

CSS

:root {
  --back-in-black: #191621;
  --back-in-black-rgb: 25 22 33;
  --back-in-black-oklch: oklch(0.209 0.022 297);
}

.element {
  color: var(--back-in-black);
  background-color: hsl(256 20% 11%);
  border-color: oklch(0.209 0.022 297);
}

Tailwind CSS v4

@theme {
  --color-back-in-black: #191621;
  --color-back-in-black-50: #f6f4fb;
  --color-back-in-black-100: #e9e6ee;
  --color-back-in-black-200: #d5d2dd;
  --color-back-in-black-300: #b9b5c1;
  --color-back-in-black-400: #9f9caa;
  --color-back-in-black-500: #878393;
  --color-back-in-black-600: #736f7d;
  --color-back-in-black-700: #5c5964;
  --color-back-in-black-800: #434149;
  --color-back-in-black-900: #2e2d33;
  --color-back-in-black-950: #1b1a1f;
}

SCSS

$back-in-black: #191621;
$back-in-black-rgb: 25, 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);
}