#12121D

CSS

:root {
  --back-in-black: #12121d;
  --back-in-black-rgb: 18 18 29;
  --back-in-black-oklch: oklch(0.188 0.022 284.1);
}

.element {
  color: var(--back-in-black);
  background-color: hsl(240 23% 9%);
  border-color: oklch(0.188 0.022 284.1);
}

Tailwind CSS v4

@theme {
  --color-back-in-black: #12121d;
  --color-back-in-black-50: #f4f4fb;
  --color-back-in-black-100: #e7e7ef;
  --color-back-in-black-200: #d3d3de;
  --color-back-in-black-300: #b6b6c3;
  --color-back-in-black-400: #9c9dac;
  --color-back-in-black-500: #848495;
  --color-back-in-black-600: #70707e;
  --color-back-in-black-700: #595a65;
  --color-back-in-black-800: #41424a;
  --color-back-in-black-900: #2d2d33;
  --color-back-in-black-950: #1a1a1f;
}

SCSS

$back-in-black: #12121d;
$back-in-black-rgb: 18, 18, 29;
$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);
}