diff --git a/products/canvas/backend/tests/test_cloud_builder.py b/products/canvas/backend/tests/test_cloud_builder.py
index 2337bc96dc5f..3691dab3d0c1 100644
--- a/products/canvas/backend/tests/test_cloud_builder.py
+++ b/products/canvas/backend/tests/test_cloud_builder.py
@@ -1,6 +1,7 @@
 import os
 import hashlib
 import tempfile
+import subprocess
 from pathlib import Path
 from typing import Any
 
@@ -10,7 +11,7 @@
 
 from parameterized import parameterized
 
-from products.canvas.backend.build_service import run_cloud_builder, validate_builder_output
+from products.canvas.backend.build_service import node_executable, run_cloud_builder, validate_builder_output
 from products.canvas.backend.presentation.serializers import CanvasSourceProjectSerializer
 from products.canvas.backend.source import synthetic_source_project, validate_source_project
 
@@ -105,6 +106,70 @@ def test_runtime_bounds_host_side_effects(self) -> None:
         self.assertIn('url.hostname.endsWith(".posthog.com")', runtime)
         self.assertIn("serialized.length>16384", runtime)
 
+    def test_runtime_applies_the_host_theme(self) -> None:
+        result = run_cloud_builder(self._project('document.body.textContent = "Hello"'))
+
+        runtime = next(file["content"] for file in result["files"] if file["path"] == "assets/canvas-runtime.js")
+        harness = """
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+
+const classes = new Set();
+const toggles = [];
+const style = {};
+const listeners = { message: [] };
+globalThis.window = globalThis;
+globalThis.parent = {};
+globalThis.location = { hash: "#theme=dark" };
+globalThis.document = {
+    readyState: "complete",
+    documentElement: {
+        classList: {
+            toggle: (name, force) => {
+                toggles.push([name, force]);
+                force ? classes.add(name) : classes.delete(name);
+            },
+        },
+        style,
+    },
+};
+globalThis.addEventListener = (type, handler) => (listeners[type] ??= []).push(handler);
+
+new Function(readFileSync(new URL("./runtime.js", import.meta.url), "utf8"))();
+
+assert.ok(classes.has("dark"), "the theme fragment did not add the dark class before connect");
+assert.equal(style.colorScheme, "dark");
+
+const bridge = new MessageChannel();
+for (const handler of listeners.message) {
+    handler({ source: globalThis.parent, data: { channel: "posthog-canvas", type: "connect" }, ports: [bridge.port2] });
+}
+bridge.port1.postMessage({ channel: "posthog-canvas", type: "set-theme", theme: "solarized" });
+bridge.port1.postMessage({ channel: "posthog-canvas", type: "set-theme" });
+bridge.port1.postMessage({ channel: "posthog-canvas", type: "set-theme", theme: "light" });
+const deadline = Date.now() + 5000;
+while (classes.has("dark") && Date.now() < deadline) {
+    await new Promise((resolve) => setTimeout(resolve, 10));
+}
+assert.ok(!classes.has("dark"), "the set-theme frame did not remove the dark class");
+assert.equal(style.colorScheme, "light");
+// Port delivery is ordered, so by the light flip the invalid frames were
+// already processed — exactly two toggles proves they were ignored, not
+// coerced to light.
+assert.deepEqual(toggles, [["dark", true], ["dark", false]]);
+bridge.port1.close();
+"""
+        with tempfile.TemporaryDirectory() as directory:
+            (Path(directory) / "runtime.js").write_text(runtime)
+            (Path(directory) / "harness.mjs").write_text(harness)
+            process = subprocess.run(
+                [node_executable(), str(Path(directory) / "harness.mjs")],
+                capture_output=True,
+                text=True,
+                timeout=30,
+            )
+        self.assertEqual(process.returncode, 0, process.stderr)
+
     def test_freezes_declared_capabilities_into_manifest(self) -> None:
         project = self._project('document.body.textContent = "Hello"')
         project["capabilities"] = {
diff --git a/products/canvas/packages/canvas_builder/build.mjs b/products/canvas/packages/canvas_builder/build.mjs
index 04f06d4e630e..4a2665d9b871 100644
--- a/products/canvas/packages/canvas_builder/build.mjs
+++ b/products/canvas/packages/canvas_builder/build.mjs
@@ -23,7 +23,7 @@ const htmlAttribute = /([a-zA-Z][\w-]*)\s*=\s*(?:"([^"]*)"|'([^']*)')/g
 const forbiddenHtml = /(?:src|href)\s*=\s*["']\s*(javascript|data:text\/html|vbscript)/i
 const extensions = ['', '.ts', '.tsx', '.js', '.jsx', '.css', '.json', '.svg', '.txt']
 const runtimePath = 'assets/canvas-runtime.js'
-const runtime = `(()=>{const channel="posthog-canvas",pending=new Map;let sequence=0,port;const post=(message)=>port?.postMessage({channel,...message});const call=(method,payload)=>new Promise((resolve,reject)=>{const id=String(++sequence);const timer=setTimeout(()=>{pending.delete(id);reject(new Error("Canvas request timed out"));},30000);pending.set(id,{resolve,reject,timer});post({type:"data-request",id,method,payload});});const receive=(event)=>{if(event.data?.channel!==channel||event.data?.type!=="data-response")return;const request=pending.get(event.data.id);if(!request)return;pending.delete(event.data.id);clearTimeout(request.timer);event.data.ok?request.resolve(event.data.result):request.reject(new Error(event.data.error??"Canvas request failed"));};const capture=(event,properties,distinctId)=>{const normalized=properties??{};let serialized;try{serialized=JSON.stringify(normalized)}catch{throw new Error("Canvas capture properties must be serializable")};if(typeof serialized!=="string"||serialized.length>16384)throw new Error("Canvas capture properties are too large");return call("capture",{event,properties:normalized,distinctId})};const openExternal=(value)=>{const url=new URL(value);if(url.protocol!=="https:"||!(url.hostname==="posthog.com"||url.hostname.endsWith(".posthog.com")))throw new Error("Canvas external URL is not allowed");post({type:"open-external",url:url.href})};window.ph={loadInsight:(shortId,options)=>call("loadInsight",{shortId,dateRange:options?.dateRange}),query:(query,params)=>call("query",typeof query==="string"?{hogql:query,params:params??{}}:{query,params:params??{}}),capture,openExternal};addEventListener("message",(event)=>{if(port||event.source!==parent||event.data?.channel!==channel||event.data?.type!=="connect"||!event.ports[0])return;port=event.ports[0];port.addEventListener("message",receive);port.start();if(document.readyState!=="loading")post({type:"ready"});if(document.readyState==="complete")post({type:"rendered"});});addEventListener("error",(event)=>post({type:"error",message:event.message||"Canvas runtime error",stack:event.error?.stack}));addEventListener("unhandledrejection",(event)=>post({type:"error",message:event.reason instanceof Error?event.reason.message:String(event.reason),stack:event.reason instanceof Error?event.reason.stack:undefined}));addEventListener("DOMContentLoaded",()=>post({type:"ready"}));addEventListener("load",()=>post({type:"rendered"}));})();`
+const runtime = `(()=>{const channel="posthog-canvas",pending=new Map;let sequence=0,port;const post=(message)=>port?.postMessage({channel,...message});const call=(method,payload)=>new Promise((resolve,reject)=>{const id=String(++sequence);const timer=setTimeout(()=>{pending.delete(id);reject(new Error("Canvas request timed out"));},30000);pending.set(id,{resolve,reject,timer});post({type:"data-request",id,method,payload});});const applyTheme=(theme)=>{if(theme!=="dark"&&theme!=="light")return;const dark=theme==="dark";document.documentElement.classList.toggle("dark",dark);document.documentElement.style.colorScheme=dark?"dark":"light";};applyTheme(new URLSearchParams(location.hash.slice(1)).get("theme"));const receive=(event)=>{if(event.data?.channel!==channel)return;if(event.data.type==="set-theme"){applyTheme(event.data.theme);return}if(event.data.type!=="data-response")return;const request=pending.get(event.data.id);if(!request)return;pending.delete(event.data.id);clearTimeout(request.timer);event.data.ok?request.resolve(event.data.result):request.reject(new Error(event.data.error??"Canvas request failed"));};const capture=(event,properties,distinctId)=>{const normalized=properties??{};let serialized;try{serialized=JSON.stringify(normalized)}catch{throw new Error("Canvas capture properties must be serializable")};if(typeof serialized!=="string"||serialized.length>16384)throw new Error("Canvas capture properties are too large");return call("capture",{event,properties:normalized,distinctId})};const openExternal=(value)=>{const url=new URL(value);if(url.protocol!=="https:"||!(url.hostname==="posthog.com"||url.hostname.endsWith(".posthog.com")))throw new Error("Canvas external URL is not allowed");post({type:"open-external",url:url.href})};window.ph={loadInsight:(shortId,options)=>call("loadInsight",{shortId,dateRange:options?.dateRange}),query:(query,params)=>call("query",typeof query==="string"?{hogql:query,params:params??{}}:{query,params:params??{}}),capture,openExternal};addEventListener("message",(event)=>{if(port||event.source!==parent||event.data?.channel!==channel||event.data?.type!=="connect"||!event.ports[0])return;port=event.ports[0];port.addEventListener("message",receive);port.start();if(document.readyState!=="loading")post({type:"ready"});if(document.readyState==="complete")post({type:"rendered"});});addEventListener("error",(event)=>post({type:"error",message:event.message||"Canvas runtime error",stack:event.error?.stack}));addEventListener("unhandledrejection",(event)=>post({type:"error",message:event.reason instanceof Error?event.reason.message:String(event.reason),stack:event.reason instanceof Error?event.reason.stack:undefined}));addEventListener("DOMContentLoaded",()=>post({type:"ready"}));addEventListener("load",()=>post({type:"rendered"}));})();`
 const platformStylesheet = `
 @import "tailwindcss";
 @import "@posthog/quill/tokens.css";