#3573D1

CSS

:root {
  --fading-night: #3573d1;
  --fading-night-rgb: 53 115 209;
  --fading-night-oklch: oklch(0.564 0.159 258.7);
}

.element {
  color: var(--fading-night);
  background-color: hsl(216 63% 51%);
  border-color: oklch(0.564 0.159 258.7);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3573d1;
  --color-fading-night-50: #dcf7ff;
  --color-fading-night-100: #cae9ff;
  --color-fading-night-200: #aed5ff;
  --color-fading-night-300: #88b8ff;
  --color-fading-night-400: #629efb;
  --color-fading-night-500: #3d83ed;
  --color-fading-night-600: #356fc8;
  --color-fading-night-700: #2a59a1;
  --color-fading-night-800: #194278;
  --color-fading-night-900: #102d55;
  --color-fading-night-950: #031a39;
}

SCSS

$fading-night: #3573d1;
$fading-night-rgb: 53, 115, 209;
$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);
}