Add dark mode docs

docs/tools/kit/index.mdx

@@ -36,6 +36,10 @@ Hooks for interacting with native Flow Cadence runtime:
 - [`useCrossVmSpendNft`](#usecrossvmspendnft) – Bridge NFTs from Cadence to Flow EVM and execute arbitrary EVM transactions to atomically spend them
 - [`useCrossVmSpendToken`](#usecrossvmspendnft) – Bridge fungible tokens from Cadence to Flow EVM and execute arbitrary EVM transactions
 
+### UI Hooks
+
+- [`useDarkMode`](#usedarkmode) – Manage dark mode state and toggle functionality
+
 ### Components
 
 Reusable UI components:
@@ -74,6 +78,7 @@ function Root() {
         appDetailUrl: "https://myonchainapp.com",
       }}
       flowJson={flowJSON}
+      enableDarkMode={true}
     >
       <App />
     </FlowProvider>
@@ -574,6 +579,37 @@ function TransactionStatusComponent() {
 
 ---
 
+### `useDarkMode`
+
+```tsx
+import { useDarkMode } from "@onflow/kit"
+```
+
+This hook provides dark mode state management and toggle functionality. It requires the `enableDarkMode` prop to be set to `true` in the `FlowProvider`.
+
+#### Returns:
+
+- `isDark: boolean` – Current dark mode state
+- `toggleDark: () => void` – Function to toggle between light and dark modes
+
+#### Example:
+
+```tsx
+import { useDarkMode } from "@onflow/kit";
+
+export default function DarkModeToggle() {
+  const { isDark, toggleDark } = useDarkMode();
+
+  return (
+    <button onClick={toggleDark}>
+      {isDark ? "🌞 Light Mode" : "🌙 Dark Mode"}
+    </button>
+  );
+}
+```
+
+---
+
 ## Cross-VM Hooks
 
 ### `useCrossVmBatchTransaction`