#3F74C6

CSS

:root {
  --fading-night: #3f74c6;
  --fading-night-rgb: 63 116 198;
  --fading-night-oklch: oklch(0.563 0.139 258.9);
}

.element {
  color: var(--fading-night);
  background-color: hsl(216 54% 51%);
  border-color: oklch(0.563 0.139 258.9);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3f74c6;
  --color-fading-night-50: #dff6ff;
  --color-fading-night-100: #cee9ff;
  --color-fading-night-200: #b3d5ff;
  --color-fading-night-300: #8fb8ff;
  --color-fading-night-400: #6a9ff0;
  --color-fading-night-500: #4985e1;
  --color-fading-night-600: #3e70be;
  --color-fading-night-700: #325a98;
  --color-fading-night-800: #204272;
  --color-fading-night-900: #152e50;
  --color-fading-night-950: #061a35;
}

SCSS

$fading-night: #3f74c6;
$fading-night-rgb: 63, 116, 198;
$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);
}