#3975C7

CSS

:root {
  --fading-night: #3975c7;
  --fading-night-rgb: 57 117 199;
  --fading-night-oklch: oklch(0.564 0.142 256.9);
}

.element {
  color: var(--fading-night);
  background-color: hsl(215 56% 50%);
  border-color: oklch(0.564 0.142 256.9);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3975c7;
  --color-fading-night-50: #ddf7ff;
  --color-fading-night-100: #cceaff;
  --color-fading-night-200: #b0d6ff;
  --color-fading-night-300: #8bb9ff;
  --color-fading-night-400: #65a0f1;
  --color-fading-night-500: #4286e2;
  --color-fading-night-600: #3971bf;
  --color-fading-night-700: #2d5b99;
  --color-fading-night-800: #1c4372;
  --color-fading-night-900: #122e50;
  --color-fading-night-950: #051a36;
}

SCSS

$fading-night: #3975c7;
$fading-night-rgb: 57, 117, 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);
}