Do not show the install prompt for themes.

chrome/test/data/extensions/api_test/webstore_private/theme.html

+<!--
+ * Copyright (c) 2012 The Chromium Authors. All rights reserved.  Use of this
+ * source code is governed by a BSD-style license that can be found in the
+ * LICENSE file.
+-->
+<script src="common.js"></script>
+<script>
+
+var themeId = 'iamefpfkojoapidjnbafmgkgncegbkad';
+
+runTests([
+  function installTheme() {
+    listenOnce(chrome.management.onInstalled, function(info) {
+      assertEq(info.id, themeId);
+    });
+
+    var manifest = getManifest("../../theme/manifest.json");
+    chrome.webstorePrivate.beginInstallWithManifest3(
+        {id: themeId, manifest: manifest},
+        callbackPass(function(result) {
+      assertEq("", result);
+      chrome.webstorePrivate.completeInstall(themeId, callbackPass());
+    }));
+  }
+]);
+
+</script>