#0F005D

CSS

:root {
  --the-vast-of-night: #0f005d;
  --the-vast-of-night-rgb: 15 0 93;
  --the-vast-of-night-oklch: oklch(0.225 0.144 272.6);
}

.element {
  color: var(--the-vast-of-night);
  background-color: hsl(250 100% 18%);
  border-color: oklch(0.225 0.144 272.6);
}

Tailwind CSS v4

@theme {
  --color-the-vast-of-night: #0f005d;
  --color-the-vast-of-night-50: #e8f3ff;
  --color-the-vast-of-night-100: #d9e5ff;
  --color-the-vast-of-night-200: #c1d0ff;
  --color-the-vast-of-night-300: #a1b1ff;
  --color-the-vast-of-night-400: #8297f3;
  --color-the-vast-of-night-500: #677ce5;
  --color-the-vast-of-night-600: #5769c1;
  --color-the-vast-of-night-700: #46549b;
  --color-the-vast-of-night-800: #303e74;
  --color-the-vast-of-night-900: #202b52;
  --color-the-vast-of-night-950: #101837;
}

SCSS

$the-vast-of-night: #0f005d;
$the-vast-of-night-rgb: 15, 0, 93;
$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);
}