diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..95ef395
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,7 @@
+# Commit Style
+
+- Use Conventional Commits.
+- Keep the type and subject lowercase.
+- Keep commit messages terse.
+
+Example: `feat: dark mode by default`
diff --git a/src/api.test.ts b/src/api.test.ts
index 5e79849..a1a1ccd 100644
--- a/src/api.test.ts
+++ b/src/api.test.ts
@@ -20,6 +20,15 @@ function withApi() {
 }
 
 describe("api", () => {
+  test("creates a drawing with dark theme by default", async () => {
+    const { api, cleanup } = withApi();
+
+    const created = await api.createDrawing().json();
+
+    expect(created.appState.theme).toBe("dark");
+    cleanup();
+  });
+
   test("returns 400 for invalid JSON", async () => {
     const { api, cleanup } = withApi();
     const drawing = await api.createDrawing().json();
diff --git a/src/db.test.ts b/src/db.test.ts
index 7c5c670..8c52d38 100644
--- a/src/db.test.ts
+++ b/src/db.test.ts
@@ -27,6 +27,7 @@ describe("drawing store", () => {
     const created = store.createDrawing();
 
     expect(created.title).toBe("Untitled");
+    expect(created.scene.appState.theme).toBe("dark");
     expect(store.listDrawings()).toHaveLength(1);
 
     const updated = store.updateDrawing(created.id, {
diff --git a/src/scene.test.ts b/src/scene.test.ts
index c9fdff7..f8f1a91 100644
--- a/src/scene.test.ts
+++ b/src/scene.test.ts
@@ -1,7 +1,13 @@
 import { describe, expect, test } from "bun:test";
-import { HttpError, MAX_SCENE_BYTES, normalizeScene, parseJsonBody } from "./scene";
+import { HttpError, MAX_SCENE_BYTES, emptyScene, normalizeScene, parseJsonBody } from "./scene";
 
 describe("scene helpers", () => {
+  test("creates empty scenes with dark theme by default", () => {
+    const scene = emptyScene();
+
+    expect(scene.appState.theme).toBe("dark");
+  });
+
   test("normalizes volatile app state fields", () => {
     const scene = normalizeScene({
       elements: [],
diff --git a/src/scene.ts b/src/scene.ts
index 8bf5d1e..32435e9 100644
--- a/src/scene.ts
+++ b/src/scene.ts
@@ -26,7 +26,7 @@ const VOLATILE_APP_STATE_KEYS = new Set([
 export function emptyScene(): ScenePayload {
   return {
     elements: [],
-    appState: {},
+    appState: { theme: "dark" },
     files: {},
   };
 }