Add smooth theme-switch transitions and restore hero .overlay wrapper

- Add prefers-reduced-motion-scoped CSS transitions (bg, color, border) on
  :root, body, and key layout containers so light/dark theme switches animate
  smoothly instead of snapping
- Restore <div class="overlay"> child in hero.php; slim .custom-hero rule to a
  customisation hook only — visual overlay styles are handled by .overlay child
- Regenerate template.min.css

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/media/css/template.css

@@ -28,6 +28,23 @@
 	}
 }
 
+/* Smooth colour transition when the data-bs-theme attribute changes between light and dark.
+   Scoped to no-preference so users who opt out of motion also skip colour flashes. */
+@media (prefers-reduced-motion: no-preference) {
+	:root,
+	body,
+	.container-wrapper,
+	.container-main,
+	nav,
+	header,
+	footer {
+		transition:
+			background-color 0.3s ease,
+			color 0.3s ease,
+			border-color 0.3s ease;
+	}
+}
+
 @media print {
 
 	nav,
@@ -14094,6 +14111,7 @@ meter {
 	border-bottom: var(--hero-border-bottom, solid var(--accent-color-secondary));
 }
 
+/* Legacy: standard Cassiopeia banner with an explicit .overlay child */
 .container-banner .banner-overlay .overlay {
 	display: -webkit-box;
 	display: -ms-flexbox;
@@ -14115,6 +14133,11 @@ meter {
 	background-color: var(--hero-overlay-bg, hsla(0, 0%, 0%, 0.5));
 }
 
+/* Hero layout (hero.php): outer wrapper — visual overlay is handled by the .overlay child */
+.container-banner .banner-overlay.custom-hero {
+	/* Selector kept as a customisation hook; see .banner-overlay .overlay for overlay styles */
+}
+
 .container-banner .banner-overlay .overlay .text-thin {
 	font-weight: lighter;
 }