diff --git a/build.py b/build.py
index e14c237..08970dd 100644
--- a/build.py
+++ b/build.py
@@ -4,68 +4,129 @@ Build a single self-contained HTML file from the source files.
 
 Usage:
     python3 build.py [output.html]
-
-If output is not given, defaults to 'background-standalone.html'.
+    python3 build.py --check          build to a temp file and only verify
 
 What gets inlined:
-  - config.js, i18n.js                       (plain <script> swap)
-  - vendor/suncalc.js, vendor/clouds3d.js    (plain <script> swap)
-  - vendor/stars.json                        (window.__starsData)
-  - every theme CSS referenced by window.THEMES
-                                             (window.__inlineThemeCss,
-                                              so theme switching works
-                                              without any files)
+  - config.js, i18n.js                        (plain <script> swap)
+  - vendor/suncalc.js, vendor/plane3d.js,
+    vendor/clouds3d.js                        (plain <script> swap)
+  - vendor/stars.json                         (window.__starsData)
+  - every stylesheet referenced by window.THEMES
+                                              (window.__inlineThemeCss, so
+                                               theme switching works offline)
 
-The result needs NO sibling files. Note that runtime DATA is still
-fetched when the page loads: basemap/elevation tiles (via the nginx
-CORS proxies of the Docker image), Overpass road + place data and the
-Open-Meteo weather API. Serve the standalone file from the same nginx
-(or a warm IndexedDB cache = fully offline) for the full experience;
-opened via file:// it still runs sky, clouds and the whole UI, just
-without the textured ground.
+The result needs no sibling files. Runtime DATA is still fetched on load:
+basemap and elevation tiles, Overpass road/settlement cells and the Open-Meteo
+weather API. Serve the file from the same nginx as the development site (its
+CORS proxies make the tiles reachable) or open it with a warm IndexedDB cache
+for fully offline rendering. Opened via file:// it still runs sky, clouds,
+terrain shading and the whole UI, just without the textured ground.
+
+The build fails loudly if any external script or stylesheet reference survives.
 """
 
 import json
 import os
 import re
 import sys
+import tempfile
 
 ROOT = os.path.dirname(os.path.abspath(__file__))
 
+INLINED_SCRIPTS = [
+    'config.js',
+    'i18n.js',
+    'vendor/suncalc.js',
+    'vendor/plane3d.js',
+    'vendor/clouds3d.js',
+]
+
 
 def read(path):
     with open(os.path.join(ROOT, path), 'r', encoding='utf-8') as f:
         return f.read()
 
 
-def guard(s):
-    # a literal </script> inside an inline block would end it early
-    return s.replace('</', '<\\/')
-
-
 def inline_script(html, tag, body):
     old = '<script src="%s"></script>' % tag
-    new = '<script>\n%s\n</script>' % body
     if old not in html:
         raise SystemExit('build.py: expected tag not found: ' + tag)
-    return html.replace(old, new, 1)
+    # a literal </script> inside an inline block would end it early
+    return html.replace(old, '<script>\n%s\n</script>' % body.replace('</', '<\\/'), 1)
 
 
 def parse_themes(config_src):
-    m = re.search(r'window\.THEMES\s*=\s*\{(.*?)\}', config_src, re.S)
+    """Return {themeId: cssFileName} from window.THEMES.
+
+    Accepts both shapes:
+        'ash': 'ash.css'
+        'ash': { css: 'ash.css', lighting: { ... } }
+    """
+    m = re.search(r'window\.THEMES\s*=\s*\{', config_src)
     if not m:
         raise SystemExit('build.py: window.THEMES not found in config.js')
-    return dict(re.findall(r"'([^']+)'\s*:\s*'([^']+\.css)'", m.group(1)))
+    i = m.end()
+    depth = 1
+    block_start = i
+    while i < len(config_src) and depth:
+        c = config_src[i]
+        if c == '{':
+            depth += 1
+        elif c == '}':
+            depth -= 1
+        i += 1
+    block = config_src[block_start:i - 1]
+
+    themes = {}
+    # top level entries only: split on commas that sit at brace depth 0
+    parts, buf, depth = [], '', 0
+    for c in block:
+        if c == '{':
+            depth += 1
+        elif c == '}':
+            depth -= 1
+        if c == ',' and depth == 0:
+            parts.append(buf)
+            buf = ''
+        else:
+            buf += c
+    parts.append(buf)
+
+    for part in parts:
+        entry = re.match(r"\s*['\"]?([A-Za-z0-9_-]+)['\"]?\s*:\s*(.*)", part, re.S)
+        if not entry:
+            continue
+        name, value = entry.group(1), entry.group(2)
+        css = re.search(r"['\"]([^'\"]+\.css)['\"]", value)
+        if not css:
+            raise SystemExit('build.py: theme %r has no .css reference' % name)
+        themes[name] = css.group(1)
+    if not themes:
+        raise SystemExit('build.py: window.THEMES is empty')
+    return themes
+
+
+def verify(html):
+    """Fail if anything still points at a sibling file."""
+    leftovers = re.findall(r'<script[^>]*\bsrc="([^"]+)"', html)
+    leftovers += [re.search(r'\bhref="([^"]+)"', tag).group(1)
+                  for tag in re.findall(r'<link[^>]*>', html)
+                  if re.search(r'\bhref="[^"]+\.css"', tag)]
+    if leftovers:
+        raise SystemExit('build.py: not self-contained, still referencing: ' +
+                         ', '.join(sorted(set(leftovers))))
 
 
 def main():
-    out = sys.argv[1] if len(sys.argv) > 1 else 'background-standalone.html'
-    html = read('background.html')
+    check = '--check' in sys.argv
+    args = [a for a in sys.argv[1:] if not a.startswith('--')]
+    out = args[0] if args else 'background-standalone.html'
 
+    html = read('background.html')
     config_src = read('config.js')
     themes = parse_themes(config_src)
 
-    # embedded data preamble (before clouds3d's own scripts run)
+    # embedded data preamble (must run before the scene scripts)
     stars = json.loads(read('vendor/stars.json'))
     theme_css = {name: read(fname) for name, fname in themes.items()}
     embed = (
@@ -73,29 +134,42 @@ def main():
         'window.__starsData = %s;\n'
         'window.__inlineThemeCss = %s;\n'
         '</script>\n'
-        % (json.dumps(stars, separators=(',', ':')),
-           json.dumps(theme_css))
+        % (json.dumps(stars, separators=(',', ':')), json.dumps(theme_css))
     )
-    anchor = '<script src="vendor/suncalc.js"></script>'
+    # must come before config.js so the early theme loader sees the CSS
+    anchor = '<script src="config.js"></script>'
+    if anchor not in html:
+        raise SystemExit('build.py: anchor script tag not found in background.html')
     html = html.replace(anchor, embed + anchor, 1)
 
-    html = inline_script(html, 'config.js', read('config.js'))
-    html = inline_script(html, 'i18n.js', read('i18n.js'))
-    html = inline_script(html, 'vendor/suncalc.js', read('vendor/suncalc.js'))
-    html = inline_script(html, 'vendor/plane3d.js', read('vendor/plane3d.js'))
-    html = inline_script(html, 'vendor/clouds3d.js', read('vendor/clouds3d.js'))
+    for tag in INLINED_SCRIPTS:
+        html = inline_script(html, tag, read(tag))
 
-    # the default <link id="theme-css" href="ash.css"> stays but is
-    # disabled by the inline-theme loader; keep a guarded fallback anyway
-    html = html.replace('<link id="theme-css"', '<!-- standalone: themes embedded -->\n    <link id="theme-css"', 1)
+    # themes are embedded in window.__inlineThemeCss, so the stylesheet link
+    # is dropped completely (the theme loader guards on getElementById)
+    rep_link = re.search(r'\n[ \t]*<link id="theme-css"[^>]*>', html)
+    if not rep_link:
+        raise SystemExit('build.py: theme-css link not found in background.html')
+    html = html[:rep_link.start()] + '\n    <!-- standalone build: themes are embedded -->' + \
+        html[rep_link.end():]
+
+    verify(html)
+
+    if check:
+        with tempfile.NamedTemporaryFile('w', suffix='.html', delete=False) as f:
+            f.write(html)
+            tmp = f.name
+        print('OK: self-contained (%d bytes, %d themes, %d stars)'
+              % (len(html), len(themes), len(stars)))
+        os.unlink(tmp)
+        return
 
     with open(os.path.join(ROOT, out), 'w', encoding='utf-8') as f:
         f.write(html)
 
     print('Wrote %s (%d bytes)' % (out, len(html)))
-    print('Inlined: config.js, i18n.js, suncalc.js, clouds3d.js,')
-    print('         stars.json (%d entries), themes: %s'
-          % (len(stars), ', '.join(themes)))
+    print('Inlined: %s, stars.json (%d entries), themes: %s'
+          % (', '.join(INLINED_SCRIPTS), len(stars), ', '.join(sorted(themes))))
 
 
 if __name__ == '__main__':