diff --git a/tests/test_script_escaping.py b/tests/test_script_escaping.py
new file mode 100644
index 0000000..5a0d05c
--- /dev/null
+++ b/tests/test_script_escaping.py
@@ -0,0 +1,90 @@
+"""Values interpolated into a <script> must be valid JavaScript.
+
+Autoescaping does not stop at HTML text nodes: it applies inside `<script>`
+too. But a classic script is not HTML-parsed, so `&#34;` never decodes back to
+a quote — it is a SyntaxError, and a syntax error takes down the *entire*
+script element.
+
+That is what `theme.style: "dark"` did. The template built the value by hand
+as `'"' + config.style + '"'`, which reached the page as
+
+    var fixed = &#34;dark&#34;;
+
+so the pre-paint IIFE failed to parse. The visible symptom was only "dark mode
+does not apply", but the same IIFE also carries the language redirect, and that
+died too — a theming option silently disabling i18n. Neither is covered by
+looking at the rendered HTML for the right *text*; the check has to be that the
+script parses.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import re
+
+import pytest
+
+from xpkgindex.build import build
+from xpkgindex.render import render
+
+from conftest import commit, init_repo, write_config, write_descriptor
+
+SCRIPT = re.compile(r"<script(?![^>]*\bsrc=)[^>]*>(.*?)</script>", re.S)
+# Entities that are meaningful in JS source and never legal inside it.
+BROKEN = re.compile(r"&(?:#34|quot|#39|apos|amp|lt|gt);")
+
+
+def _site(tmp_path, **config):
+    root = str(tmp_path / "index")
+    os.makedirs(root)
+    init_repo(root)
+    write_config(root, **config)
+    write_descriptor(root, "alpha", "widget")
+    commit(root, "add widget", date="2026-01-01")
+    out = str(tmp_path / "site")
+    site, config = build(root, offline=True)
+    render(site, config, out)
+    return out
+
+
+def _inline_scripts(site):
+    for dirpath, _, names in os.walk(site):
+        for n in names:
+            if not n.endswith(".html"):
+                continue
+            path = os.path.join(dirpath, n)
+            with open(path, encoding="utf-8") as f:
+                html = f.read()
+            for m in SCRIPT.finditer(html):
+                yield os.path.relpath(path, site), m.group(1)
+
+
+@pytest.mark.parametrize("style", ["auto", "dark", "light"])
+def test_inline_scripts_carry_no_html_entities(tmp_path, style):
+    site = _site(tmp_path, theme={"style": style})
+    for page, body in _inline_scripts(site):
+        found = BROKEN.search(body)
+        assert not found, (
+            f"{page}: inline script contains {found.group(0)!r}; HTML entities "
+            f"do not decode inside <script>, so this is a SyntaxError that "
+            f"disables every statement in the element"
+        )
+
+
+@pytest.mark.parametrize("style,expected", [("dark", "dark"), ("light", "light")])
+def test_fixed_theme_reaches_the_page_as_a_js_string(tmp_path, style, expected):
+    site = _site(tmp_path, theme={"style": style})
+    with open(os.path.join(site, "index.html"), encoding="utf-8") as f:
+        html = f.read()
+    m = re.search(r"var fixed = (.+?);", html)
+    assert m, "the pre-paint theme bootstrap is gone"
+    assert json.loads(m.group(1)) == expected
+
+
+def test_auto_leaves_the_theme_to_the_visitor(tmp_path):
+    site = _site(tmp_path, theme={"style": "auto"})
+    with open(os.path.join(site, "index.html"), encoding="utf-8") as f:
+        html = f.read()
+    m = re.search(r"var fixed = (.+?);", html)
+    assert m and json.loads(m.group(1)) is None
diff --git a/xpkgindex/templates/base.html b/xpkgindex/templates/base.html
index 8ffd7b9..d43ed7a 100644
--- a/xpkgindex/templates/base.html
+++ b/xpkgindex/templates/base.html
@@ -22,7 +22,14 @@
   (function () {
     try {
       var pref = localStorage.getItem('xpi-theme');
-      var fixed = {{ ('"' + config.style + '"') if config.style in ('light', 'dark') else 'null' }};
+      {# `tojson`, not a hand-built quoted string: autoescaping applies inside
+         <script> too, so `'"' + style + '"'` reached the page as
+         `var fixed = &#34;dark&#34;;`. HTML entities are not decoded in a
+         classic script, so that is a SyntaxError -- and a syntax error takes
+         the whole IIFE with it, including the language redirect below, which
+         has nothing to do with theming. `tojson` emits `"dark"` or `null` and
+         escapes for a script context. #}
+      var fixed = {{ (config.style if config.style in ('light', 'dark') else None) | tojson }};
       document.documentElement.dataset.theme = pref || fixed ||
         (matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light');
     } catch (e) { /* private mode: fall through to the served markup */ }