diff --git a/server/shell.ts b/server/shell.ts
index a44602c..f1b6939 100644
--- a/server/shell.ts
+++ b/server/shell.ts
@@ -470,7 +470,9 @@ export function renderExternalAppShell(opts: ExternalAppShellOptions): string {
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>🌌</text></svg>">
   <title>${escapeHtml(title)}</title>
-  <link rel="stylesheet" href="/shell.css?v=6">
+  <script>(function(){var t=localStorage.getItem('canvas-theme');if(!t)t=matchMedia('(prefers-color-scheme:light)').matches?'light':'dark';document.documentElement.setAttribute('data-theme',t)})()</script>
+  <link rel="stylesheet" href="/theme.css?v=1">
+  <link rel="stylesheet" href="/shell.css?v=7">
   <style>
     html.rspace-embedded .rstack-header { display: none !important; }
     html.rspace-embedded .rstack-tab-row { display: none !important; }
@@ -478,9 +480,8 @@ export function renderExternalAppShell(opts: ExternalAppShellOptions): string {
   </style>
   <script>if (window.self !== window.parent) document.documentElement.classList.add('rspace-embedded');</script>
 </head>
-<body data-theme="${theme}">
-  <script>(function(){try{var t=localStorage.getItem('canvas-theme');if(t)document.body.setAttribute('data-theme',t)}catch(e){}})()</script>
-  <header class="rstack-header" data-theme="${theme}">
+<body>
+  <header class="rstack-header">
     <div class="rstack-header__left">
       <a href="/" style="display:flex;align-items:center;margin-right:4px"><img src="/favicon.png" alt="rSpace" class="rstack-header__logo"></a>
       <rstack-app-switcher current="${escapeAttr(moduleId)}"></rstack-app-switcher>
@@ -494,7 +495,7 @@ export function renderExternalAppShell(opts: ExternalAppShellOptions): string {
       <rstack-identity></rstack-identity>
     </div>
   </header>
-  <div class="rstack-tab-row" data-theme="${theme}">
+  <div class="rstack-tab-row">
     <rstack-tab-bar space="${escapeAttr(spaceSlug)}" active="" view-mode="flat"></rstack-tab-bar>
   </div>
   <div class="rspace-iframe-wrap">
@@ -514,8 +515,7 @@ export function renderExternalAppShell(opts: ExternalAppShellOptions): string {
   </div>
 
   <script type="module">
-    import '/shell.js?v=6';
-    (function(){try{var t=localStorage.getItem('canvas-theme');if(t)document.querySelectorAll('[data-theme]').forEach(function(el){el.setAttribute('data-theme',t)})}catch(e){}})();
+    import '/shell.js?v=7';
     document.querySelector('rstack-app-switcher')?.setModules(${moduleListJSON});
 
     const tabBar = document.querySelector('rstack-tab-bar');
@@ -723,13 +723,14 @@ export function renderModuleLanding(opts: ModuleLandingOptions): string {
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>${mod.icon}</text></svg>">
   <title>${escapeHtml(mod.name)} — rSpace</title>
-  <link rel="stylesheet" href="/shell.css?v=6">
+  <script>(function(){var t=localStorage.getItem('canvas-theme');if(!t)t=matchMedia('(prefers-color-scheme:light)').matches?'light':'dark';document.documentElement.setAttribute('data-theme',t)})()</script>
+  <link rel="stylesheet" href="/theme.css?v=1">
+  <link rel="stylesheet" href="/shell.css?v=7">
   ${cssBlock}
   <script defer src="https://rdata.online/collect.js" data-website-id="6ee7917b-0ed7-44cb-a4c8-91037638526b"></script>
 </head>
-<body data-theme="${theme}">
-  <script>(function(){try{var t=localStorage.getItem('canvas-theme');if(t)document.body.setAttribute('data-theme',t)}catch(e){}})()</script>
-  <header class="rstack-header" data-theme="${theme}">
+<body>
+  <header class="rstack-header">
     <div class="rstack-header__left">
       <a href="/" style="display:flex;align-items:center;margin-right:4px"><img src="/favicon.png" alt="rSpace" class="rstack-header__logo"></a>
       <rstack-app-switcher current="${escapeAttr(mod.id)}"></rstack-app-switcher>
@@ -744,8 +745,7 @@ export function renderModuleLanding(opts: ModuleLandingOptions): string {
   </header>
   ${bodyContent}
   <script type="module">
-    import '/shell.js?v=6';
-    (function(){try{var t=localStorage.getItem('canvas-theme');if(t)document.querySelectorAll('[data-theme]').forEach(function(el){el.setAttribute('data-theme',t)})}catch(e){}})();
+    import '/shell.js?v=7';
     document.querySelector('rstack-app-switcher')?.setModules(${moduleListJSON});
     function _updateDemoBtn() {
       var btn = document.querySelector('.rstack-header__demo-btn');
diff --git a/shared/components/rstack-identity.ts b/shared/components/rstack-identity.ts
index b1e2c0a..7bbe01a 100644
--- a/shared/components/rstack-identity.ts
+++ b/shared/components/rstack-identity.ts
@@ -1266,8 +1266,7 @@ export class RStackIdentity extends HTMLElement {
 					e.stopPropagation();
 					const newTheme = themeToggle.checked ? "dark" : "light";
 					localStorage.setItem("canvas-theme", newTheme);
-					document.body.setAttribute("data-theme", newTheme);
-					document.querySelectorAll(".rstack-header, .rstack-tab-row").forEach(el => el.setAttribute("data-theme", newTheme));
+					document.documentElement.setAttribute("data-theme", newTheme);
 					this.dispatchEvent(new CustomEvent("theme-change", { bubbles: true, composed: true, detail: { theme: newTheme } }));
 					this.#render();
 				});
diff --git a/website/public/theme.css b/website/public/theme.css
new file mode 100644
index 0000000..bc09b22
--- /dev/null
+++ b/website/public/theme.css
@@ -0,0 +1,193 @@
+/* ── rSpace Theme System ──
+ * CSS custom properties for dark/light theming.
+ * Dark is default on :root; light overrides on [data-theme="light"].
+ * prefers-color-scheme fallback applies when no data-theme is set yet. */
+
+:root {
+  color-scheme: dark;
+
+  /* Surface */
+  --rs-bg-page:            #0f172a;
+  --rs-bg-surface:         #1e293b;
+  --rs-bg-surface-raised:  #334155;
+  --rs-bg-surface-sunken:  #0f172a;
+  --rs-bg-overlay:         rgba(15, 23, 42, 0.85);
+  --rs-bg-hover:           rgba(255, 255, 255, 0.05);
+  --rs-bg-active:          rgba(6, 182, 212, 0.1);
+
+  /* Text */
+  --rs-text-primary:       #e2e8f0;
+  --rs-text-secondary:     #94a3b8;
+  --rs-text-muted:         #64748b;
+  --rs-text-inverse:       #0f172a;
+
+  /* Borders */
+  --rs-border:             rgba(255, 255, 255, 0.1);
+  --rs-border-subtle:      rgba(255, 255, 255, 0.06);
+  --rs-border-strong:      rgba(255, 255, 255, 0.2);
+
+  /* Glass */
+  --rs-glass-bg:           rgba(15, 23, 42, 0.85);
+  --rs-glass-border:       rgba(255, 255, 255, 0.08);
+
+  /* Shadows */
+  --rs-shadow-sm:          0 1px 3px rgba(0, 0, 0, 0.3);
+  --rs-shadow-md:          0 4px 20px rgba(0, 0, 0, 0.4);
+  --rs-shadow-lg:          0 8px 30px rgba(0, 0, 0, 0.4);
+
+  /* Spinner */
+  --rs-spinner-track:      rgba(255, 255, 255, 0.1);
+  --rs-spinner-head:       #14b8a6;
+
+  /* Accent (same both themes) */
+  --rs-accent:             #14b8a6;
+  --rs-accent-hover:       #0d9488;
+
+  /* Primary (same both themes) */
+  --rs-primary:            #4f46e5;
+  --rs-primary-hover:      #6366f1;
+
+  /* Semantic (same both themes) */
+  --rs-error:              #ef4444;
+  --rs-success:            #22c55e;
+  --rs-warning:            #fbbf24;
+
+  /* Gradients (same both themes) */
+  --rs-gradient-brand:     linear-gradient(135deg, #14b8a6, #22d3ee);
+  --rs-gradient-primary:   linear-gradient(135deg, #6366f1, #4f46e5);
+  --rs-gradient-cta:       linear-gradient(135deg, #14b8a6, #0d9488);
+
+  /* Component-specific tokens */
+  --rs-input-bg:           #0f172a;
+  --rs-input-border:       #334155;
+  --rs-input-text:         #e2e8f0;
+  --rs-btn-secondary-bg:   rgba(255, 255, 255, 0.08);
+  --rs-btn-secondary-text: #94a3b8;
+  --rs-card-bg:            rgba(255, 255, 255, 0.03);
+  --rs-card-border:        rgba(255, 255, 255, 0.06);
+
+  /* Canvas */
+  --rs-canvas-bg:          #0f172a;
+  --rs-canvas-grid:        rgba(255, 255, 255, 0.04);
+
+  /* Toolbar */
+  --rs-toolbar-bg:         #1e293b;
+  --rs-toolbar-btn-bg:     #334155;
+  --rs-toolbar-btn-hover:  #475569;
+  --rs-toolbar-btn-text:   #e2e8f0;
+  --rs-toolbar-sep:        #334155;
+  --rs-toolbar-panel-bg:   #1e293b;
+  --rs-toolbar-panel-border: #334155;
+}
+
+[data-theme="light"] {
+  color-scheme: light;
+
+  /* Surface */
+  --rs-bg-page:            #f8fafc;
+  --rs-bg-surface:         #ffffff;
+  --rs-bg-surface-raised:  #f1f5f9;
+  --rs-bg-surface-sunken:  #f8fafc;
+  --rs-bg-overlay:         rgba(255, 255, 255, 0.9);
+  --rs-bg-hover:           rgba(0, 0, 0, 0.04);
+  --rs-bg-active:          #e0f2fe;
+
+  /* Text */
+  --rs-text-primary:       #0f172a;
+  --rs-text-secondary:     #374151;
+  --rs-text-muted:         #64748b;
+  --rs-text-inverse:       #e2e8f0;
+
+  /* Borders */
+  --rs-border:             rgba(0, 0, 0, 0.1);
+  --rs-border-subtle:      rgba(0, 0, 0, 0.06);
+  --rs-border-strong:      rgba(0, 0, 0, 0.2);
+
+  /* Glass */
+  --rs-glass-bg:           rgba(255, 255, 255, 0.9);
+  --rs-glass-border:       rgba(0, 0, 0, 0.08);
+
+  /* Shadows */
+  --rs-shadow-sm:          0 1px 3px rgba(0, 0, 0, 0.05);
+  --rs-shadow-md:          0 4px 20px rgba(0, 0, 0, 0.15);
+  --rs-shadow-lg:          0 8px 30px rgba(0, 0, 0, 0.18);
+
+  /* Spinner */
+  --rs-spinner-track:      rgba(0, 0, 0, 0.08);
+  --rs-spinner-head:       #14b8a6;
+
+  /* Component-specific tokens */
+  --rs-input-bg:           #f8fafc;
+  --rs-input-border:       #e2e8f0;
+  --rs-input-text:         #334155;
+  --rs-btn-secondary-bg:   rgba(0, 0, 0, 0.05);
+  --rs-btn-secondary-text: #374151;
+  --rs-card-bg:            rgba(0, 0, 0, 0.02);
+  --rs-card-border:        rgba(0, 0, 0, 0.06);
+
+  /* Canvas */
+  --rs-canvas-bg:          #ffffff;
+  --rs-canvas-grid:        #f1f5f9;
+
+  /* Toolbar */
+  --rs-toolbar-bg:         #ffffff;
+  --rs-toolbar-btn-bg:     #f1f5f9;
+  --rs-toolbar-btn-hover:  #e2e8f0;
+  --rs-toolbar-btn-text:   #0f172a;
+  --rs-toolbar-sep:        #e2e8f0;
+  --rs-toolbar-panel-bg:   #ffffff;
+  --rs-toolbar-panel-border: #e2e8f0;
+}
+
+/* prefers-color-scheme fallback: if JS hasn't set data-theme yet */
+@media (prefers-color-scheme: light) {
+  :root:not([data-theme]) {
+    color-scheme: light;
+
+    --rs-bg-page:            #f8fafc;
+    --rs-bg-surface:         #ffffff;
+    --rs-bg-surface-raised:  #f1f5f9;
+    --rs-bg-surface-sunken:  #f8fafc;
+    --rs-bg-overlay:         rgba(255, 255, 255, 0.9);
+    --rs-bg-hover:           rgba(0, 0, 0, 0.04);
+    --rs-bg-active:          #e0f2fe;
+
+    --rs-text-primary:       #0f172a;
+    --rs-text-secondary:     #374151;
+    --rs-text-muted:         #64748b;
+    --rs-text-inverse:       #e2e8f0;
+
+    --rs-border:             rgba(0, 0, 0, 0.1);
+    --rs-border-subtle:      rgba(0, 0, 0, 0.06);
+    --rs-border-strong:      rgba(0, 0, 0, 0.2);
+
+    --rs-glass-bg:           rgba(255, 255, 255, 0.9);
+    --rs-glass-border:       rgba(0, 0, 0, 0.08);
+
+    --rs-shadow-sm:          0 1px 3px rgba(0, 0, 0, 0.05);
+    --rs-shadow-md:          0 4px 20px rgba(0, 0, 0, 0.15);
+    --rs-shadow-lg:          0 8px 30px rgba(0, 0, 0, 0.18);
+
+    --rs-spinner-track:      rgba(0, 0, 0, 0.08);
+    --rs-spinner-head:       #14b8a6;
+
+    --rs-input-bg:           #f8fafc;
+    --rs-input-border:       #e2e8f0;
+    --rs-input-text:         #334155;
+    --rs-btn-secondary-bg:   rgba(0, 0, 0, 0.05);
+    --rs-btn-secondary-text: #374151;
+    --rs-card-bg:            rgba(0, 0, 0, 0.02);
+    --rs-card-border:        rgba(0, 0, 0, 0.06);
+
+    --rs-canvas-bg:          #ffffff;
+    --rs-canvas-grid:        #f1f5f9;
+
+    --rs-toolbar-bg:         #ffffff;
+    --rs-toolbar-btn-bg:     #f1f5f9;
+    --rs-toolbar-btn-hover:  #e2e8f0;
+    --rs-toolbar-btn-text:   #0f172a;
+    --rs-toolbar-sep:        #e2e8f0;
+    --rs-toolbar-panel-bg:   #ffffff;
+    --rs-toolbar-panel-border: #e2e8f0;
+  }
+}
