#0D1830

CSS

:root {
  --sombre-night: #0d1830;
  --sombre-night-rgb: 13 24 48;
  --sombre-night-oklch: oklch(0.213 0.05 264.1);
}

.element {
  color: var(--sombre-night);
  background-color: hsl(221 57% 12%);
  border-color: oklch(0.213 0.05 264.1);
}

Tailwind CSS v4

@theme {
  --color-sombre-night: #0d1830;
  --color-sombre-night-50: #eef5ff;
  --color-sombre-night-100: #e0e8f9;
  --color-sombre-night-200: #cad4ea;
  --color-sombre-night-300: #abb7d2;
  --color-sombre-night-400: #909fbd;
  --color-sombre-night-500: #7586a8;
  --color-sombre-night-600: #63728e;
  --color-sombre-night-700: #4f5b72;
  --color-sombre-night-800: #394354;
  --color-sombre-night-900: #272e3b;
  --color-sombre-night-950: #151b25;
}

SCSS

$sombre-night: #0d1830;
$sombre-night-rgb: 13, 24, 48;
$sombre-night-light: mix(#fff, $sombre-night, 20%);
$sombre-night-dark: mix(#000, $sombre-night, 20%);

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