diff --git a/index.html b/index.html
index ba20d7b..bfc4425 100644
--- a/index.html
+++ b/index.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="ja">
+<html lang="en">
 
 <head>
     <meta charset="UTF-8">
@@ -14,7 +14,7 @@
             scroll-behavior: smooth;
         }
 
-        /* カスタムスクロールバー */
+        /* Custom scrollbar */
         ::-webkit-scrollbar {
             width: 6px;
         }
@@ -24,12 +24,12 @@
         }
 
         ::-webkit-scrollbar-thumb {
-            background: #e2e8f0;
+            background: #4b5563;
             border-radius: 10px;
         }
 
         ::-webkit-scrollbar-thumb:hover {
-            background: #cbd5e1;
+            background: #6b7280;
         }
 
         .message-animation {
@@ -50,40 +50,41 @@
     </style>
 </head>
 
-<body class="bg-gray-50 text-gray-900 font-sans h-screen flex flex-col overflow-hidden">
+<body class="bg-gray-900 text-gray-100 font-sans h-screen flex flex-col overflow-hidden">
 
-    <!-- ヘッダー -->
-    <header class="flex items-center justify-between px-6 py-4 bg-white border-b shadow-sm z-10">
+    <!-- Header -->
+    <header class="flex items-center justify-between px-6 py-4 bg-gray-800 border-b border-gray-700 shadow-sm z-10">
         <div class="flex items-center gap-2">
             <div class="p-2 bg-indigo-600 rounded-lg shadow-blue-200 shadow-lg">
                 <i data-lucide="bot" class="text-white w-6 h-6"></i>
             </div>
             <div>
-                <h1 class="text-lg font-bold tracking-tight text-gray-800 leading-none">AI Assistant</h1>
+                <h1 class="text-lg font-bold tracking-tight text-gray-100 leading-none">AI Assistant</h1>
                 <span class="text-[10px] text-green-500 font-medium uppercase tracking-tighter">● Online</span>
             </div>
         </div>
-        <button id="clear-btn" class="p-2 text-gray-400 hover:text-red-500 hover:bg-red-50 rounded-full transition-all"
-            title="チャットをクリア">
+        <button id="clear-btn"
+            class="p-2 text-gray-400 hover:text-red-400 hover:bg-gray-700 rounded-full transition-all"
+            title="Clear chat">
             <i data-lucide="trash-2" class="w-5 h-5"></i>
         </button>
     </header>
 
-    <!-- メッセージ表示エリア -->
+    <!-- Message display area -->
     <main id="chat-window" class="flex-1 overflow-y-auto px-4 py-8 scroll-smooth">
-        <div id="messages-container" class="max-w-3xl mx-auto space-y-6">
-            <!-- メッセージがここに挿入されます -->
+        <div id="messages-container" class="w-full space-y-6">
+            <!-- Messages will be inserted here -->
         </div>
 
-        <!-- ローディング表示（初期状態は非表示） -->
-        <div id="loading-indicator" class="hidden max-w-3xl mx-auto mt-6">
+        <!-- Loading indicator (initially hidden) -->
+        <div id="loading-indicator" class="hidden w-full mt-6">
             <div class="flex gap-4">
                 <div
-                    class="flex-shrink-0 w-8 h-8 rounded-full bg-white border flex items-center justify-center shadow-sm">
+                    class="flex-shrink-0 w-8 h-8 rounded-full bg-gray-700 border border-gray-600 flex items-center justify-center shadow-sm">
                     <i data-lucide="bot" class="text-indigo-400 w-4 h-4"></i>
                 </div>
                 <div
-                    class="bg-white border border-gray-100 px-4 py-3 rounded-2xl rounded-tl-none shadow-sm flex items-center gap-3">
+                    class="bg-gray-800 border border-gray-700 px-4 py-3 rounded-2xl rounded-tl-none shadow-sm flex items-center gap-3">
                     <div class="flex gap-1">
                         <span
                             class="w-1.5 h-1.5 bg-indigo-400 rounded-full animate-bounce [animation-delay:-0.3s]"></span>
@@ -91,24 +92,24 @@
                             class="w-1.5 h-1.5 bg-indigo-400 rounded-full animate-bounce [animation-delay:-0.15s]"></span>
                         <span class="w-1.5 h-1.5 bg-indigo-400 rounded-full animate-bounce"></span>
                     </div>
-                    <span class="text-gray-400 text-xs font-medium">AI応答生成中...</span>
+                    <span class="text-gray-400 text-xs font-medium">AI is thinking...</span>
                 </div>
             </div>
         </div>
 
-        <!-- エラー表示エリア -->
-        <div id="error-container" class="hidden max-w-2xl mx-auto mt-6">
-            <!-- エラー内容がここに挿入されます -->
+        <!-- Error display area -->
+        <div id="error-container" class="hidden w-full mt-6">
+            <!-- Error content will be inserted here -->
         </div>
     </main>
 
-    <!-- 入力エリア -->
-    <footer class="bg-white border-t p-4">
-        <div class="max-w-3xl mx-auto">
+    <!-- Input area -->
+    <footer class="bg-gray-800 border-t border-gray-700 p-4">
+        <div class="w-full px-2">
             <div
-                class="relative flex items-end gap-2 bg-gray-50 border border-gray-200 rounded-2xl p-1.5 focus-within:ring-2 focus-within:ring-indigo-500 focus-within:bg-white transition-all shadow-inner">
-                <textarea id="user-input" placeholder="質問を入力してください..." rows="1"
-                    class="flex-1 bg-transparent border-none focus:ring-0 text-sm py-2 px-3 resize-none max-h-40 min-h-[40px] outline-none"></textarea>
+                class="relative flex items-end gap-2 bg-gray-700 border border-gray-600 rounded-2xl p-1.5 focus-within:ring-2 focus-within:ring-indigo-500 focus-within:bg-gray-700 transition-all shadow-inner">
+                <textarea id="user-input" placeholder="Type your message..." rows="1"
+                    class="flex-1 bg-transparent border-none focus:ring-0 text-sm py-2 px-3 resize-none max-h-40 min-h-[40px] outline-none text-gray-100 placeholder-gray-400"></textarea>
                 <button id="send-btn" class="p-2.5 rounded-xl transition-all text-gray-300 disabled:cursor-not-allowed"
                     disabled>
                     <i data-lucide="send" id="send-icon" class="w-5 h-5"></i>
@@ -128,7 +129,7 @@
         const MODEL = 'openai/gpt-oss-20b';
 
         let messages = [
-            { role: 'assistant', content: 'こんにちは！何かお手伝いできることはありますか？' }
+            { role: 'assistant', content: 'Hello! How can I help you today?' }
         ];
         let isLoading = false;
 
@@ -155,11 +156,11 @@
                 const isUser = msg.role === 'user';
                 const messageHtml = `
                     <div class="flex gap-4 ${isUser ? 'flex-row-reverse' : 'flex-row'} message-animation">
-                        <div class="flex-shrink-0 w-8 h-8 rounded-full flex items-center justify-center shadow-sm ${isUser ? 'bg-indigo-600' : 'bg-white border'}">
-                            <i data-lucide="${isUser ? 'user' : 'bot'}" class="${isUser ? 'text-white' : 'text-indigo-600'} w-4 h-4"></i>
+                        <div class="flex-shrink-0 w-8 h-8 rounded-full flex items-center justify-center shadow-sm ${isUser ? 'bg-indigo-600' : 'bg-gray-700 border border-gray-600'}">
+                            <i data-lucide="${isUser ? 'user' : 'bot'}" class="${isUser ? 'text-white' : 'text-indigo-400'} w-4 h-4"></i>
                         </div>
                         <div class="flex flex-col max-w-[85%] ${isUser ? 'items-end' : 'items-start'}">
-                            <div class="px-4 py-2.5 rounded-2xl shadow-sm text-sm ${isUser ? 'bg-indigo-600 text-white rounded-tr-none' : 'bg-white text-gray-800 border border-gray-100 rounded-tl-none'}">
+                            <div class="px-4 py-2.5 rounded-2xl shadow-sm text-sm ${isUser ? 'bg-indigo-600 text-white rounded-tr-none' : 'bg-gray-800 text-gray-100 border border-gray-700 rounded-tl-none'}">
                                 <p class="whitespace-pre-wrap break-words">${msg.content}</p>
                             </div>
                         </div>
@@ -177,16 +178,16 @@
 
         function showError(message) {
             errorContainer.innerHTML = `
-                <div class="flex flex-col gap-2 p-4 bg-red-50 border border-red-100 rounded-xl text-red-700">
+                <div class="flex flex-col gap-2 p-4 bg-red-900 bg-opacity-30 border border-red-800 rounded-xl text-red-400">
                     <div class="flex items-center gap-2 font-bold text-sm">
                         <i data-lucide="alert-circle" class="w-4 h-4"></i>
-                        <span>エラーが発生しました</span>
+                        <span>An error occurred</span>
                     </div>
                     <p class="text-xs opacity-90 leading-relaxed">${message}</p>
                     ${message.includes('Failed to fetch') ? `
-                        <div class="mt-2 text-[10px] bg-red-100 p-2 rounded flex gap-2 items-start">
+                        <div class="mt-2 text-[10px] bg-red-950 bg-opacity-50 p-2 rounded flex gap-2 items-start">
                             <i data-lucide="info" class="w-3 h-3 mt-0.5 flex-shrink-0"></i>
-                            <span>ヒント: 外部ドメインからのAPI呼び出しがブラウザにブロックされています。サーバー側で Access-Control-Allow-Origin ヘッダーを設定するか、開発中は CORS 解除用のブラウザ拡張機能を試してください。</span>
+                            <span>Hint: API calls from external domains are being blocked by the browser. Set the Access-Control-Allow-Origin header on the server side, or try using a CORS-disabling browser extension during development.</span>
                         </div>
                     ` : ''}
                 </div>
@@ -229,18 +230,18 @@
 
                 if (!response.ok) {
                     const errorData = await response.json().catch(() => ({}));
-                    throw new Error(errorData.error?.message || `APIエラー (ステータス: ${response.status})`);
+                    throw new Error(errorData.error?.message || `API error (status: ${response.status})`);
                 }
 
                 const data = await response.json();
                 if (data.choices && data.choices[0]?.message) {
                     messages.push(data.choices[0].message);
                 } else {
-                    throw new Error('レスポンスの形式が正しくありません。');
+                    throw new Error('Invalid response format.');
                 }
             } catch (err) {
                 console.error(err);
-                showError(err.message === 'Failed to fetch' ? 'APIへの接続に失敗しました。CORS制限を確認してください。' : err.message);
+                showError(err.message === 'Failed to fetch' ? 'Failed to connect to API. Please check CORS restrictions.' : err.message);
             } finally {
                 isLoading = false;
                 updateUIState();
@@ -280,12 +281,12 @@
         sendBtn.addEventListener('click', handleSend);
 
         clearBtn.addEventListener('click', () => {
-            messages = [{ role: 'assistant', content: 'チャットをリセットしました。何かお手伝いしましょうか？' }];
+            messages = [{ role: 'assistant', content: 'Chat has been reset. How can I help you?' }];
             errorContainer.classList.add('hidden');
             renderMessages();
         });
 
-        // 初期表示
+        // Initial render
         renderMessages();
         initIcons();
     </script>
