#3F75C3

CSS

:root {
  --fading-night: #3f75c3;
  --fading-night-rgb: 63 117 195;
  --fading-night-oklch: oklch(0.564 0.134 257.7);
}

.element {
  color: var(--fading-night);
  background-color: hsl(215 52% 51%);
  border-color: oklch(0.564 0.134 257.7);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3f75c3;
  --color-fading-night-50: #dff7ff;
  --color-fading-night-100: #ceeaff;
  --color-fading-night-200: #b3d6ff;
  --color-fading-night-300: #8fb8fc;
  --color-fading-night-400: #6a9fed;
  --color-fading-night-500: #4986dd;
  --color-fading-night-600: #3e71bb;
  --color-fading-night-700: #325b96;
  --color-fading-night-800: #204370;
  --color-fading-night-900: #152e4f;
  --color-fading-night-950: #061b34;
}

SCSS

$fading-night: #3f75c3;
$fading-night-rgb: 63, 117, 195;
$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);
}