#151723

CSS

:root {
  --back-in-black: #151723;
  --back-in-black-rgb: 21 23 35;
  --back-in-black-oklch: oklch(0.209 0.024 276.8);
}

.element {
  color: var(--back-in-black);
  background-color: hsl(231 25% 11%);
  border-color: oklch(0.209 0.024 276.8);
}

Tailwind CSS v4

@theme {
  --color-back-in-black: #151723;
  --color-back-in-black-50: #f3f5fc;
  --color-back-in-black-100: #e6e7f0;
  --color-back-in-black-200: #d2d3df;
  --color-back-in-black-300: #b4b6c4;
  --color-back-in-black-400: #9a9dad;
  --color-back-in-black-500: #828596;
  --color-back-in-black-600: #6e717f;
  --color-back-in-black-700: #585a66;
  --color-back-in-black-800: #40424b;
  --color-back-in-black-900: #2c2d34;
  --color-back-in-black-950: #191a1f;
}

SCSS

$back-in-black: #151723;
$back-in-black-rgb: 21, 23, 35;
$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);
}