#3B78C3

CSS

:root {
  --fading-night: #3b78c3;
  --fading-night-rgb: 59 120 195;
  --fading-night-oklch: oklch(0.569 0.133 255);
}

.element {
  color: var(--fading-night);
  background-color: hsl(213 54% 50%);
  border-color: oklch(0.569 0.133 255);
}

Tailwind CSS v4

@theme {
  --color-fading-night: #3b78c3;
  --color-fading-night-50: #def7ff;
  --color-fading-night-100: #cceaff;
  --color-fading-night-200: #b0d7ff;
  --color-fading-night-300: #8bbafb;
  --color-fading-night-400: #66a1eb;
  --color-fading-night-500: #4287dc;
  --color-fading-night-600: #3973b9;
  --color-fading-night-700: #2d5c95;
  --color-fading-night-800: #1d446f;
  --color-fading-night-900: #132f4e;
  --color-fading-night-950: #051b34;
}

SCSS

$fading-night: #3b78c3;
$fading-night-rgb: 59, 120, 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);
}