Add dark mode

frontend/src/index.css

@@ -10,6 +10,30 @@ body,
   padding: 0;
 }
 
+html.dark {
+  background-color: #1c1917;
+  color-scheme: dark;
+}
+
+/* Smooth theme transitions (scoped to avoid map performance issues) */
+body,
+div,
+aside,
+section,
+header,
+nav,
+button,
+input,
+select,
+label,
+span,
+p,
+h1,
+h2,
+h3 {
+  transition: background-color 0.2s ease, border-color 0.2s ease, color 0.2s ease;
+}
+
 /* Fade-in animation for homepage sections */
 .fade-in-section {
   opacity: 0;