#12005C

CSS

:root {
  --the-vast-of-night: #12005c;
  --the-vast-of-night-rgb: 18 0 92;
  --the-vast-of-night-oklch: oklch(0.226 0.142 274.7);
}

.element {
  color: var(--the-vast-of-night);
  background-color: hsl(252 100% 18%);
  border-color: oklch(0.226 0.142 274.7);
}

Tailwind CSS v4

@theme {
  --color-the-vast-of-night: #12005c;
  --color-the-vast-of-night-50: #eaf2ff;
  --color-the-vast-of-night-100: #dbe4ff;
  --color-the-vast-of-night-200: #c4cfff;
  --color-the-vast-of-night-300: #a4b1ff;
  --color-the-vast-of-night-400: #8696f2;
  --color-the-vast-of-night-500: #6c7be3;
  --color-the-vast-of-night-600: #5b68bf;
  --color-the-vast-of-night-700: #49539a;
  --color-the-vast-of-night-800: #323d73;
  --color-the-vast-of-night-900: #222a51;
  --color-the-vast-of-night-950: #121736;
}

SCSS

$the-vast-of-night: #12005c;
$the-vast-of-night-rgb: 18, 0, 92;
$the-vast-of-night-light: mix(#fff, $the-vast-of-night, 20%);
$the-vast-of-night-dark: mix(#000, $the-vast-of-night, 20%);

.element {
  color: $the-vast-of-night;
  box-shadow: 0 0 0 2px rgba($the-vast-of-night, 0.35);
}