diff --git a/apps/zen.nix b/apps/zen.nix
index 1cdb812..6170191 100644
--- a/apps/zen.nix
+++ b/apps/zen.nix
@@ -9,48 +9,79 @@ let
   # <profile>/chrome/zen-theme.css as a style-sheet-service USER_SHEET here
   # and re-register it under a cache-busting URI whenever its mtime moves —
   # that is what makes a `theme-apply` switch land in a running Zen
-  # (settings/theming.nix writes that file on every switch). A separate file,
-  # not userChrome.css: the legacy loader would hold its own copy of the same
-  # sheet and the two would race on cascade order.
+  # (settings/theming.nix writes that file on every switch).
+  #
+  # userChrome.css can't stay as a safety net: nsIStyleSheetService user
+  # sheets are ordered BEFORE the profile's userChrome.css, so a leftover copy
+  # would outrank every sheet we register and pin Zen to whichever wallpaper
+  # was live at startup — the exact bug this fixes.
+  #
+  # Everything here goes through Components.classes. The autoconfig sandbox is
+  # not a normal system-principal scope: `Services` is not a global in it, and
+  # reaching for it throws before a single sheet is registered.
+  #
+  # Diagnosing a silent failure: about:config -> fred.zenTheme.status holds
+  # "ok <mtime>", "missing <path>" or "error <exception>" from the last pass.
   themeHotReload = ''
     try {
-      const { classes: kCc, interfaces: kCi } = Components;
+      const kCc = Components.classes;
+      const kCi = Components.interfaces;
       const sss = kCc["@mozilla.org/content/style-sheet-service;1"]
                     .getService(kCi.nsIStyleSheetService);
+      const io = kCc["@mozilla.org/network/io-service;1"]
+                   .getService(kCi.nsIIOService);
+      const dirs = kCc["@mozilla.org/file/directory_service;1"]
+                     .getService(kCi.nsIProperties);
+      const prefs = kCc["@mozilla.org/preferences-service;1"]
+                      .getService(kCi.nsIPrefBranch);
+
+      const note = (msg) => {
+        try { prefs.setStringPref("fred.zenTheme.status", msg); } catch (e) {}
+      };
 
-      const file = Services.dirsvc.get("ProfD", kCi.nsIFile);
-      file.append("chrome");
-      file.append("zen-theme.css");
-      const base = Services.io.newFileURI(file).spec;
       let live = null;
 
       const apply = () => {
-        if (!file.exists()) return;
-        // install(1) stamps the copy with the current time, so mtime alone
-        // distinguishes one wallpaper's sheet from the next.
-        const stamp = String(file.lastModifiedTime);
-        if (stamp === live) return;
-        // A fresh query string dodges the CSS loader's per-URI cache; without
-        // it a re-register would hand back the sheet parsed at startup.
-        sss.loadAndRegisterSheet(Services.io.newURI(base + "?" + stamp),
-                                 sss.USER_SHEET);
-        if (live !== null) {
-          const old = Services.io.newURI(base + "?" + live);
-          if (sss.sheetRegistered(old, sss.USER_SHEET))
-            sss.unregisterSheet(old, sss.USER_SHEET);
+        try {
+          // Resolved per pass, not once: autoconfig runs early enough that
+          // ProfD is not guaranteed yet, and a throw here would kill the timer
+          // for the whole session.
+          const file = dirs.get("ProfD", kCi.nsIFile);
+          file.append("chrome");
+          file.append("zen-theme.css");
+          if (!file.exists()) { note("missing " + file.path); return; }
+
+          // install(1) stamps the copy with the current time, so mtime alone
+          // distinguishes one wallpaper's sheet from the next.
+          const stamp = String(file.lastModifiedTime);
+          if (stamp === live) return;
+
+          // A fresh query string dodges the CSS loader's per-URI cache; without
+          // it a re-register would hand back the sheet parsed at startup.
+          const base = io.newFileURI(file).spec;
+          sss.loadAndRegisterSheet(io.newURI(base + "?" + stamp), sss.USER_SHEET);
+          if (live !== null) {
+            const old = io.newURI(base + "?" + live);
+            if (sss.sheetRegistered(old, sss.USER_SHEET))
+              sss.unregisterSheet(old, sss.USER_SHEET);
+          }
+          live = stamp;
+          note("ok " + stamp);
+        } catch (e) {
+          note("error " + e);
         }
-        live = stamp;
       };
 
       const timer = kCc["@mozilla.org/timer;1"].createInstance(kCi.nsITimer);
       // The observer service holds the only strong reference that keeps the
       // timer (and this closure) off the GC's list for the session.
-      Services.obs.addObserver({ timer, observe() {} }, "quit-application");
+      kCc["@mozilla.org/observer-service;1"].getService(kCi.nsIObserverService)
+        .addObserver({ timer, observe() {} }, "quit-application");
       timer.initWithCallback({ notify: apply }, 2000,
                              kCi.nsITimer.TYPE_REPEATING_SLACK);
       apply();
     } catch (e) {
-      Components.utils.reportError(e);
+      Components.utils.reportError("zen-theme: " + e);
     }
   '';
 in