#0D005E

CSS

:root {
  --the-vast-of-night: #0d005e;
  --the-vast-of-night-rgb: 13 0 94;
  --the-vast-of-night-oklch: oklch(0.225 0.146 271.2);
}

.element {
  color: var(--the-vast-of-night);
  background-color: hsl(248 100% 18%);
  border-color: oklch(0.225 0.146 271.2);
}

Tailwind CSS v4

@theme {
  --color-the-vast-of-night: #0d005e;
  --color-the-vast-of-night-50: #e7f3ff;
  --color-the-vast-of-night-100: #d7e5ff;
  --color-the-vast-of-night-200: #bfd1ff;
  --color-the-vast-of-night-300: #9fb2ff;
  --color-the-vast-of-night-400: #7f98f4;
  --color-the-vast-of-night-500: #647ce6;
  --color-the-vast-of-night-600: #556ac2;
  --color-the-vast-of-night-700: #43549c;
  --color-the-vast-of-night-800: #2e3e75;
  --color-the-vast-of-night-900: #1f2b52;
  --color-the-vast-of-night-950: #0f1837;
}

SCSS

$the-vast-of-night: #0d005e;
$the-vast-of-night-rgb: 13, 0, 94;
$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);
}