#3578C7

CSS

:root {
  --fading-night: #3578c7;
  --fading-night-rgb: 53 120 199;
  --fading-night-oklch: oklch(0.569 0.14 254.5);
}

.element {
  color: var(--fading-night);
  background-color: hsl(212 58% 49%);
  border-color: oklch(0.569 0.14 254.5);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3578c7;
  --color-fading-night-50: #dcf8ff;
  --color-fading-night-100: #caebff;
  --color-fading-night-200: #aed7ff;
  --color-fading-night-300: #88bafe;
  --color-fading-night-400: #61a1ef;
  --color-fading-night-500: #3c87e0;
  --color-fading-night-600: #3473bd;
  --color-fading-night-700: #295c98;
  --color-fading-night-800: #194471;
  --color-fading-night-900: #102f50;
  --color-fading-night-950: #031b35;
}

SCSS

$fading-night: #3578c7;
$fading-night-rgb: 53, 120, 199;
$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);
}