#0F015C

CSS

:root {
  --the-vast-of-night: #0f015c;
  --the-vast-of-night-rgb: 15 1 92;
  --the-vast-of-night-oklch: oklch(0.225 0.142 272.7);
}

.element {
  color: var(--the-vast-of-night);
  background-color: hsl(249 98% 18%);
  border-color: oklch(0.225 0.142 272.7);
}

Tailwind CSS v4

@theme {
  --color-the-vast-of-night: #0f015c;
  --color-the-vast-of-night-50: #e8f3ff;
  --color-the-vast-of-night-100: #d9e5ff;
  --color-the-vast-of-night-200: #c2d0ff;
  --color-the-vast-of-night-300: #a2b2ff;
  --color-the-vast-of-night-400: #8297f2;
  --color-the-vast-of-night-500: #687ce3;
  --color-the-vast-of-night-600: #5869c0;
  --color-the-vast-of-night-700: #46549a;
  --color-the-vast-of-night-800: #303e73;
  --color-the-vast-of-night-900: #212b51;
  --color-the-vast-of-night-950: #101836;
}

SCSS

$the-vast-of-night: #0f015c;
$the-vast-of-night-rgb: 15, 1, 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);
}