#3F73C9

CSS

:root {
  --fading-night: #3f73c9;
  --fading-night-rgb: 63 115 201;
  --fading-night-oklch: oklch(0.563 0.145 259.9);
}

.element {
  color: var(--fading-night);
  background-color: hsl(217 56% 52%);
  border-color: oklch(0.563 0.145 259.9);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3f73c9;
  --color-fading-night-50: #dff6ff;
  --color-fading-night-100: #cee9ff;
  --color-fading-night-200: #b3d5ff;
  --color-fading-night-300: #8eb7ff;
  --color-fading-night-400: #6a9ef3;
  --color-fading-night-500: #4984e4;
  --color-fading-night-600: #3f70c1;
  --color-fading-night-700: #32599b;
  --color-fading-night-800: #204273;
  --color-fading-night-900: #152d51;
  --color-fading-night-950: #061a36;
}

SCSS

$fading-night: #3f73c9;
$fading-night-rgb: 63, 115, 201;
$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);
}