#03131B

CSS

:root {
  --black-knight: #03131b;
  --black-knight-rgb: 3 19 27;
  --black-knight-oklch: oklch(0.176 0.029 230.3);
}

.element {
  color: var(--black-knight);
  background-color: hsl(200 80% 6%);
  border-color: oklch(0.176 0.029 230.3);
}

Tailwind CSS v4

@theme {
  --color-black-knight: #03131b;
  --color-black-knight-50: #edf7fc;
  --color-black-knight-100: #dfeaf0;
  --color-black-knight-200: #c9d7df;
  --color-black-knight-300: #a9bac4;
  --color-black-knight-400: #8ea2ac;
  --color-black-knight-500: #738a96;
  --color-black-knight-600: #62757f;
  --color-black-knight-700: #4e5d65;
  --color-black-knight-800: #38454a;
  --color-black-knight-900: #272f34;
  --color-black-knight-950: #151c1f;
}

SCSS

$black-knight: #03131b;
$black-knight-rgb: 3, 19, 27;
$black-knight-light: mix(#fff, $black-knight, 20%);
$black-knight-dark: mix(#000, $black-knight, 20%);

.element {
  color: $black-knight;
  box-shadow: 0 0 0 2px rgba($black-knight, 0.35);
}