#12192A

CSS

:root {
  --sombre-night: #12192a;
  --sombre-night-rgb: 18 25 42;
  --sombre-night-oklch: oklch(0.216 0.035 266.4);
}

.element {
  color: var(--sombre-night);
  background-color: hsl(223 40% 12%);
  border-color: oklch(0.216 0.035 266.4);
}

Tailwind CSS v4

@theme {
  --color-sombre-night: #12192a;
  --color-sombre-night-50: #f1f5ff;
  --color-sombre-night-100: #e3e8f4;
  --color-sombre-night-200: #ced4e4;
  --color-sombre-night-300: #b0b7ca;
  --color-sombre-night-400: #959eb4;
  --color-sombre-night-500: #7b869e;
  --color-sombre-night-600: #687186;
  --color-sombre-night-700: #535b6b;
  --color-sombre-night-800: #3c424f;
  --color-sombre-night-900: #292e37;
  --color-sombre-night-950: #171b22;
}

SCSS

$sombre-night: #12192a;
$sombre-night-rgb: 18, 25, 42;
$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);
}