Update plvt_inject_animation_duration for fade effect

Author: mukeshpanchal27

plugins/view-transitions/includes/theme.php

@@ -382,7 +382,8 @@ function plvt_inject_animation_duration( string $css, int $animation_duration ):
 	// Inject animation duration as CSS variable to take effect.
 	$css .= sprintf(
 		/* translators: %s is animation duration in seconds. */
-		'::view-transition-group(*) { --plvt-view-transition-animation-duration: %ss; }',
+		'::view-transition-group(*) { %s: %ss; }',
+		'' !== $css ? '--plvt-view-transition-animation-duration' : 'animation-duration',
 		$seconds
 	);