#3173D3

CSS

:root {
  --fading-night: #3173d3;
  --fading-night-rgb: 49 115 211;
  --fading-night-oklch: oklch(0.564 0.162 258.2);
}

.element {
  color: var(--fading-night);
  background-color: hsl(216 65% 51%);
  border-color: oklch(0.564 0.162 258.2);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3173d3;
  --color-fading-night-50: #dbf7ff;
  --color-fading-night-100: #c9e9ff;
  --color-fading-night-200: #acd6ff;
  --color-fading-night-300: #86b8ff;
  --color-fading-night-400: #5f9efd;
  --color-fading-night-500: #3983ef;
  --color-fading-night-600: #316fca;
  --color-fading-night-700: #2759a2;
  --color-fading-night-800: #164279;
  --color-fading-night-900: #0e2d55;
  --color-fading-night-950: #021a3a;
}

SCSS

$fading-night: #3173d3;
$fading-night-rgb: 49, 115, 211;
$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);
}