#3674D6

CSS

:root {
  --fading-night: #3674d6;
  --fading-night-rgb: 54 116 214;
  --fading-night-oklch: oklch(0.57 0.164 259.3);
}

.element {
  color: var(--fading-night);
  background-color: hsl(217 66% 53%);
  border-color: oklch(0.57 0.164 259.3);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3674d6;
  --color-fading-night-50: #dbf6ff;
  --color-fading-night-100: #cae9ff;
  --color-fading-night-200: #add5ff;
  --color-fading-night-300: #88b7ff;
  --color-fading-night-400: #619dfe;
  --color-fading-night-500: #3c82f0;
  --color-fading-night-600: #346ecb;
  --color-fading-night-700: #2958a3;
  --color-fading-night-800: #18417a;
  --color-fading-night-900: #102d56;
  --color-fading-night-950: #03193a;
}

SCSS

$fading-night: #3674d6;
$fading-night-rgb: 54, 116, 214;
$fading-night-light: mix(#fff, $fading-night, 20%);
$fading-night-dark: mix(#000, $fading-night, 20%);

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