#15131B

CSS

:root {
  --back-in-black: #15131b;
  --back-in-black-rgb: 21 19 27;
  --back-in-black-oklch: oklch(0.193 0.016 296.3);
}

.element {
  color: var(--back-in-black);
  background-color: hsl(255 17% 9%);
  border-color: oklch(0.193 0.016 296.3);
}

Tailwind CSS v4

@theme {
  --color-back-in-black: #15131b;
  --color-back-in-black-50: #f5f4f9;
  --color-back-in-black-100: #e8e7ed;
  --color-back-in-black-200: #d5d3da;
  --color-back-in-black-300: #b8b6bf;
  --color-back-in-black-400: #9f9da7;
  --color-back-in-black-500: #878490;
  --color-back-in-black-600: #72707a;
  --color-back-in-black-700: #5b5961;
  --color-back-in-black-800: #424148;
  --color-back-in-black-900: #2e2d32;
  --color-back-in-black-950: #1b1a1e;
}

SCSS

$back-in-black: #15131b;
$back-in-black-rgb: 21, 19, 27;
$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);
}