#3973CD

CSS

:root {
  --fading-night: #3973cd;
  --fading-night-rgb: 57 115 205;
  --fading-night-oklch: oklch(0.563 0.152 259);
}

.element {
  color: var(--fading-night);
  background-color: hsl(216 60% 51%);
  border-color: oklch(0.563 0.152 259);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3973cd;
  --color-fading-night-50: #ddf7ff;
  --color-fading-night-100: #cce9ff;
  --color-fading-night-200: #b0d5ff;
  --color-fading-night-300: #8bb8ff;
  --color-fading-night-400: #659ef7;
  --color-fading-night-500: #4283e9;
  --color-fading-night-600: #3970c4;
  --color-fading-night-700: #2d599e;
  --color-fading-night-800: #1c4276;
  --color-fading-night-900: #122d53;
  --color-fading-night-950: #051a38;
}

SCSS

$fading-night: #3973cd;
$fading-night-rgb: 57, 115, 205;
$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);
}