diff --git a/docs/EMBEDDED_EDITOR.md b/docs/EMBEDDED_EDITOR.md
index 039504d..69ea2e9 100644
--- a/docs/EMBEDDED_EDITOR.md
+++ b/docs/EMBEDDED_EDITOR.md
@@ -78,19 +78,20 @@ full-screen overlay containing an `<iframe>` whose `src` is the editor URL
 
 ### Service worker
 
-The static editor ships a `preview-sw.js` service worker, but **the embedded
-editor never registers it**: `editor/index.php` shims
-`navigator.serviceWorker.register` so any request for `preview-sw.js` resolves to
-a no-op instead of registering (`editor/index.php:224-241`). This avoids the
-console-spamming registration errors seen where the `static.php` router is proxied
-or cached (e.g. moodle-playground) and returns a 404 for that path.
-
-Because registration is blocked, `editor/static.php` **does not** emit a
-`Service-Worker-Allowed: /` header when serving `preview-sw.js`. That header only
-widens the scope a service worker is *allowed to control*; with no registration it
-was unused, and broadcasting an unnecessarily broad (`/`) control scope is avoided.
-Removing it changes no working flow — the worker is never activated — and narrows
-the surface a future or proxied registration could claim.
+The static editor renders its preview through a `preview-sw.js` service worker,
+registered from `editor/static.php/<cmid>/preview-sw.js` with the scope
+`editor/static.php/<cmid>/viewer/`. The scope sits under the script's own path, so
+no `Service-Worker-Allowed` header is needed and `editor/static.php` does not send
+one.
+
+`editor/index.php` wraps `navigator.serviceWorker.register` only to absorb a
+**failed** registration: where the `static.php` router is proxied or cached (e.g.
+moodle-playground) the worker script 404s, and the wrapper resolves a stub instead
+of letting the error reach the console. The editor then falls back to a `blob:`
+preview. That fallback is degraded: the inlined theme CSS keeps relative `url(...)`
+references that cannot resolve from a `blob:` URL, so theme icons are missing
+(exelearning/exelearning issue 2476). Blocking the registration unconditionally, as
+earlier versions did, forced that fallback everywhere.
 
 ### Protocol messages
 
diff --git a/editor/index.php b/editor/index.php
index 0296366..40a8817 100644
--- a/editor/index.php
+++ b/editor/index.php
@@ -222,11 +222,14 @@ enumerable: true,
     // the Yjs theme bind and leaves the editor unresponsive. WP and Omeka-S
     // ship the same workaround: swallow 404s on .css / idevices URLs and
     // return an empty stylesheet so the editor keeps booting.
-    // Disable any new service-worker registration (the static editor's
-    // preview-sw.js is served from the same static.php router; environments
-    // that proxy or cache that router — e.g. moodle-playground — return a
-    // 404 there and the registration error spams the console without
-    // blocking anything).
+    // Let the static editor register its preview-sw.js: the preview renders
+    // through it, and without it the editor falls back to a blob: URL whose
+    // inlined theme CSS keeps relative url(...) references (icons, sprites)
+    // that cannot resolve, so the preview loses the theme images
+    // (exelearning/exelearning issue 2476). Only a failed registration is
+    // absorbed: environments that proxy or cache the static.php router (e.g.
+    // moodle-playground) 404 the worker script, and the resolved stub keeps
+    // that error out of the console while the editor uses its blob fallback.
     (function() {
         if ("serviceWorker" in navigator) {
             try {
@@ -234,12 +237,13 @@ enumerable: true,
                     ? navigator.serviceWorker.register.bind(navigator.serviceWorker)
                     : null;
                 navigator.serviceWorker.register = function(scriptURL, options) {
-                    if (typeof scriptURL === "string" && scriptURL.indexOf("preview-sw.js") !== -1) {
+                    if (!registerOriginal) {
                         return Promise.resolve({ scope: "" });
                     }
-                    return registerOriginal
-                        ? registerOriginal(scriptURL, options)
-                        : Promise.resolve({ scope: "" });
+                    return registerOriginal(scriptURL, options).catch(function(err) {
+                        console.warn("[mod_exelearning] Service worker registration failed:", scriptURL, err);
+                        return { scope: "" };
+                    });
                 };
             } catch (e) {
                 // Some embeds make navigator.serviceWorker non-writable; ignore.
diff --git a/tests/js/editor_service_worker_shim.test.js b/tests/js/editor_service_worker_shim.test.js
new file mode 100644
index 0000000..56d8f5b
--- /dev/null
+++ b/tests/js/editor_service_worker_shim.test.js
@@ -0,0 +1,60 @@
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+// Service-worker shim inlined by editor/index.php (exelearning/exelearning issue 2476).
+//
+// The editor's preview needs its preview-sw.js: without it the preview falls back
+// to a blob: URL where the theme CSS url(...) images (navigation icons, sprites)
+// cannot resolve. The shim must let that registration through and only absorb a
+// failed one (a proxied static.php router that 404s the worker script).
+const fs = require('fs');
+const path = require('path');
+
+/** Extracts the service-worker block of the bootstrap script and runs it. */
+function installShim(navigator, console) {
+    const source = fs.readFileSync(path.join(__dirname, '../../editor/index.php'), 'utf8');
+    const start = source.indexOf('if ("serviceWorker" in navigator) {');
+    const end = source.indexOf('var originalFetch = window.fetch;', start);
+    expect(start).toBeGreaterThan(-1);
+    expect(end).toBeGreaterThan(start);
+    new Function('navigator', 'console', source.slice(start, end))(navigator, console);
+}
+
+function fakeNavigator(register) {
+    return { serviceWorker: { register } };
+}
+
+describe('editor bootstrap: preview service worker', () => {
+    it('registers preview-sw.js so the preview can resolve theme images', async () => {
+        const registration = { scope: '/mod/exelearning/editor/static.php/2/viewer/' };
+        const register = vi.fn(() => Promise.resolve(registration));
+        const nav = fakeNavigator(register);
+        installShim(nav, console);
+
+        const result = await nav.serviceWorker.register('/static.php/2/preview-sw.js', { scope: 'viewer/' });
+
+        expect(register).toHaveBeenCalledWith('/static.php/2/preview-sw.js', { scope: 'viewer/' });
+        expect(result).toBe(registration);
+    });
+
+    it('resolves a failed registration quietly so the editor uses its blob fallback', async () => {
+        const nav = fakeNavigator(() => Promise.reject(new TypeError('404 fetching the script')));
+        const warn = vi.fn();
+        installShim(nav, { warn });
+
+        await expect(nav.serviceWorker.register('/static.php/2/preview-sw.js')).resolves.toEqual({ scope: '' });
+        expect(warn).toHaveBeenCalled();
+    });
+});
diff --git a/vitest.config.mjs b/vitest.config.mjs
index f160928..5f5e076 100644
--- a/vitest.config.mjs
+++ b/vitest.config.mjs
@@ -1,7 +1,8 @@
 import { defineConfig } from 'vitest/config';
 
 // Vitest config for the plugin's own JavaScript unit tests. Scope is deliberately
-// narrow: only the grade-critical SCORM tracker (js/scorm_tracker.js). UI glue
+// narrow: the grade-critical SCORM tracker (js/scorm_tracker.js) and the editor's
+// service-worker shim (editor/index.php). Other UI glue
 // (amd/src/fullscreen.js, resize.js, editor_modal.js, ...) and the vendored pipwerks
 // wrappers (assets/scorm/*) are out of scope. The embedded editor (exelearning/) ships
 // its own Vitest suite and is not retested here.