#000C53

CSS

:root {
  --interstellar-blue: #000c53;
  --interstellar-blue-rgb: 0 12 83;
  --interstellar-blue-oklch: oklch(0.218 0.126 264);
}

.element {
  color: var(--interstellar-blue);
  background-color: hsl(231 100% 16%);
  border-color: oklch(0.218 0.126 264);
}

Tailwind CSS v4

@theme {
  --color-interstellar-blue: #000c53;
  --color-interstellar-blue-50: #e4f5ff;
  --color-interstellar-blue-100: #d4e8ff;
  --color-interstellar-blue-200: #bbd4ff;
  --color-interstellar-blue-300: #99b6f9;
  --color-interstellar-blue-400: #789de9;
  --color-interstellar-blue-500: #5b83d9;
  --color-interstellar-blue-600: #4d6fb7;
  --color-interstellar-blue-700: #3d5993;
  --color-interstellar-blue-800: #29416e;
  --color-interstellar-blue-900: #1c2d4d;
  --color-interstellar-blue-950: #0c1a33;
}

SCSS

$interstellar-blue: #000c53;
$interstellar-blue-rgb: 0, 12, 83;
$interstellar-blue-light: mix(#fff, $interstellar-blue, 20%);
$interstellar-blue-dark: mix(#000, $interstellar-blue, 20%);

.element {
  color: $interstellar-blue;
  box-shadow: 0 0 0 2px rgba($interstellar-blue, 0.35);
}