diff --git a/scripts/dev-portal/index.html b/scripts/dev-portal/index.html
index 135ae0ac..bad25cbf 100644
--- a/scripts/dev-portal/index.html
+++ b/scripts/dev-portal/index.html
@@ -66,9 +66,25 @@
             </svg>
             HyperDX Dev Portal
           </h1>
-          <div class="status">
-            <div class="dot"></div>
-            <span id="refresh-status">Auto-refreshing every 3s</span>
+
+          <div class="header-right">
+            <div class="status">
+              <div class="dot"></div>
+              <span id="refresh-status">Auto-refreshing every 3s</span>
+            </div>
+            <button
+              class="theme-toggle"
+              id="theme-toggle"
+              onclick="toggleTheme()"
+              title="Toggle light/dark mode"
+            >
+              <svg id="theme-icon-sun" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="2">
+                <path stroke-linecap="round" stroke-linejoin="round" d="M12 3v1m0 16v1m8.66-13.66l-.71.71M4.05 19.95l-.71.71M21 12h-1M4 12H3m16.66 7.66l-.71-.71M4.05 4.05l-.71-.71M16 12a4 4 0 11-8 0 4 4 0 018 0z"/>
+              </svg>
+              <svg id="theme-icon-moon" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="2" style="display:none">
+                <path stroke-linecap="round" stroke-linejoin="round" d="M21 12.79A9 9 0 1111.21 3a7 7 0 009.79 9.79z"/>
+              </svg>
+            </button>
           </div>
         </div>
 
@@ -120,6 +136,46 @@
     </div>
 
     <script>
+      // --- Theme ---
+      function getPreferredTheme() {
+        const stored = localStorage.getItem('hdx-dev-portal-theme');
+        if (stored === 'light' || stored === 'dark') return stored;
+        return window.matchMedia('(prefers-color-scheme: light)').matches
+          ? 'light'
+          : 'dark';
+      }
+
+      function applyTheme(theme) {
+        document.documentElement.setAttribute('data-theme', theme);
+        const sunIcon = document.getElementById('theme-icon-sun');
+        const moonIcon = document.getElementById('theme-icon-moon');
+        if (sunIcon && moonIcon) {
+          // Sun icon shown in dark mode (click to go light), moon in light mode
+          sunIcon.style.display = theme === 'dark' ? '' : 'none';
+          moonIcon.style.display = theme === 'light' ? '' : 'none';
+        }
+      }
+
+      function toggleTheme() {
+        const current =
+          document.documentElement.getAttribute('data-theme') || 'dark';
+        const next = current === 'dark' ? 'light' : 'dark';
+        localStorage.setItem('hdx-dev-portal-theme', next);
+        applyTheme(next);
+      }
+
+      // Apply immediately to avoid flash
+      applyTheme(getPreferredTheme());
+
+      // Listen for OS theme changes (only when no explicit preference saved)
+      window
+        .matchMedia('(prefers-color-scheme: light)')
+        .addEventListener('change', () => {
+          if (!localStorage.getItem('hdx-dev-portal-theme')) {
+            applyTheme(getPreferredTheme());
+          }
+        });
+
       const contentEl = document.getElementById('content');
       const historyEl = document.getElementById('history-content');
       const errorEl = document.getElementById('error-banner');
diff --git a/scripts/dev-portal/styles.css b/scripts/dev-portal/styles.css
index d2425d71..72eb1bf2 100644
--- a/scripts/dev-portal/styles.css
+++ b/scripts/dev-portal/styles.css
@@ -15,6 +15,36 @@
   --orange: #db6d28;
   --log-bg: #141517; /* dark-8 */
   --hover: #25262b; /* dark-6: color-bg-hover */
+  --log-text: #c9d1d9;
+  --row-hover: rgba(88, 166, 255, 0.05);
+  --row-active: rgba(88, 166, 255, 0.1);
+  --table-border: rgba(48, 54, 61, 0.5);
+  --modal-backdrop: rgba(0, 0, 0, 0.6);
+  --hover-overlay: rgba(255, 255, 255, 0.05);
+}
+
+:root[data-theme='light'] {
+  --bg: #ffffff;
+  --card-bg: #f8f9fa;
+  --card-surface: #f1f3f5;
+  --border: #dee2e6;
+  --border-emphasis: #ced4da;
+  --text: #212529;
+  --text-muted: #868e96;
+  --accent: #12b886;
+  --accent-hover: #0ca678;
+  --green: #12b886;
+  --red: #e03131;
+  --yellow: #f08c00;
+  --orange: #e8590c;
+  --log-bg: #f8f9fa;
+  --hover: #e9ecef;
+  --log-text: #212529;
+  --row-hover: rgba(0, 0, 0, 0.03);
+  --row-active: rgba(0, 0, 0, 0.06);
+  --table-border: rgba(0, 0, 0, 0.08);
+  --modal-backdrop: rgba(0, 0, 0, 0.3);
+  --hover-overlay: rgba(0, 0, 0, 0.05);
 }
 
 * {
@@ -120,7 +150,7 @@ body {
   white-space: pre-wrap;
   word-break: break-all;
   background: var(--bg);
-  color: #c9d1d9;
+  color: var(--log-text);
 }
 
 .log-content .log-line {
@@ -166,6 +196,12 @@ body {
   flex-shrink: 0;
 }
 
+.header .header-right {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+}
+
 .header .status {
   font-size: 13px;
   color: var(--text-muted);
@@ -329,7 +365,7 @@ body {
 .services-table td {
   padding: 10px 20px;
   font-size: 14px;
-  border-bottom: 1px solid rgba(48, 54, 61, 0.5);
+  border-bottom: 1px solid var(--table-border);
 }
 
 .services-table tr:last-child td {
@@ -342,11 +378,11 @@ body {
 }
 
 .services-table tr.clickable:hover {
-  background: rgba(88, 166, 255, 0.05);
+  background: var(--row-hover);
 }
 
 .services-table tr.active {
-  background: rgba(88, 166, 255, 0.1);
+  background: var(--row-active);
 }
 
 .service-name {
@@ -517,7 +553,7 @@ body {
 
 .history-toggle-btn:hover {
   color: var(--text);
-  background: rgba(255, 255, 255, 0.05);
+  background: var(--hover-overlay);
 }
 
 .history-card-body {
@@ -543,7 +579,7 @@ body {
   justify-content: space-between;
   padding: 12px 20px;
   background: var(--card-surface);
-  border-bottom: 1px solid rgba(48, 54, 61, 0.5);
+  border-bottom: 1px solid var(--table-border);
 }
 
 .history-entry-header .history-meta {
@@ -595,11 +631,11 @@ body {
 }
 
 .file-item:hover {
-  background: rgba(88, 166, 255, 0.05);
+  background: var(--row-hover);
 }
 
 .file-item.active {
-  background: rgba(88, 166, 255, 0.1);
+  background: var(--row-active);
 }
 
 .file-name {
@@ -660,7 +696,7 @@ body {
 .modal-overlay {
   position: fixed;
   inset: 0;
-  background: rgba(0, 0, 0, 0.6);
+  background: var(--modal-backdrop);
   display: flex;
   align-items: center;
   justify-content: center;
@@ -735,3 +771,29 @@ body {
 .modal-danger:hover {
   opacity: 0.85;
 }
+
+/* --- Theme toggle --- */
+.theme-toggle {
+  background: none;
+  border: 1px solid var(--border);
+  color: var(--text-muted);
+  width: 32px;
+  height: 32px;
+  border-radius: 6px;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  transition: all 0.15s;
+  flex-shrink: 0;
+}
+
+.theme-toggle:hover {
+  color: var(--text);
+  border-color: var(--text-muted);
+}
+
+.theme-toggle svg {
+  width: 16px;
+  height: 16px;
+}