#0A005F

CSS

:root {
  --the-vast-of-night: #0a005f;
  --the-vast-of-night-rgb: 10 0 95;
  --the-vast-of-night-oklch: oklch(0.225 0.148 269.4);
}

.element {
  color: var(--the-vast-of-night);
  background-color: hsl(246 100% 19%);
  border-color: oklch(0.225 0.148 269.4);
}

Tailwind CSS v4

@theme {
  --color-the-vast-of-night: #0a005f;
  --color-the-vast-of-night-50: #e5f4ff;
  --color-the-vast-of-night-100: #d6e6ff;
  --color-the-vast-of-night-200: #bdd1ff;
  --color-the-vast-of-night-300: #9cb3ff;
  --color-the-vast-of-night-400: #7c99f6;
  --color-the-vast-of-night-500: #607de7;
  --color-the-vast-of-night-600: #516ac3;
  --color-the-vast-of-night-700: #41559d;
  --color-the-vast-of-night-800: #2c3f75;
  --color-the-vast-of-night-900: #1d2b53;
  --color-the-vast-of-night-950: #0e1837;
}

SCSS

$the-vast-of-night: #0a005f;
$the-vast-of-night-rgb: 10, 0, 95;
$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);
}