#0F172A

CSS

:root {
  --sombre-night: #0f172a;
  --sombre-night-rgb: 15 23 42;
  --sombre-night-oklch: oklch(0.208 0.04 265.8);
}

.element {
  color: var(--sombre-night);
  background-color: hsl(222 47% 11%);
  border-color: oklch(0.208 0.04 265.8);
}

Tailwind CSS v4

@theme {
  --color-sombre-night: #0f172a;
  --color-sombre-night-50: #f0f5ff;
  --color-sombre-night-100: #e2e8f6;
  --color-sombre-night-200: #cdd4e6;
  --color-sombre-night-300: #aeb7cd;
  --color-sombre-night-400: #939eb7;
  --color-sombre-night-500: #7a86a1;
  --color-sombre-night-600: #677288;
  --color-sombre-night-700: #525b6d;
  --color-sombre-night-800: #3b4251;
  --color-sombre-night-900: #292e38;
  --color-sombre-night-950: #171b23;
}

SCSS

$sombre-night: #0f172a;
$sombre-night-rgb: 15, 23, 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);
}