diff --git a/lib/shared/theme.js b/lib/shared/theme.js
index ae14422e..1de50550 100644
--- a/lib/shared/theme.js
+++ b/lib/shared/theme.js
@@ -196,17 +196,50 @@ export class ThemeManagerBase extends GObject.Object {
     if (this._needUpdate()) {
       let originalCss = this.configMgr.defaultStylesheetFile;
       let configCss = this.configMgr.stylesheetFile;
-      let copyConfigCss = Gio.File.new_for_path(this.configMgr.stylesheetFileName + ".bak");
-      let backupFine = configCss.copy(copyConfigCss, Gio.FileCopyFlags.OVERWRITE, null, null);
-      let copyFine = originalCss.copy(configCss, Gio.FileCopyFlags.OVERWRITE, null, null);
-      if (backupFine && copyFine) {
-        this.settings.set_uint("css-last-update", this.cssTag);
-        return true;
+      if (!originalCss || !configCss) return false;
+      // The backup goes next to the user's stylesheet (it went to "undefined.bak" in the
+      // working directory, #266)
+      let copyConfigCss = Gio.File.new_for_path(`${configCss.get_path()}.bak`);
+      try {
+        let backupFine = this._copyFile(configCss, copyConfigCss);
+        let copyFine = this._copyFile(originalCss, configCss);
+        if (backupFine && copyFine) {
+          this.settings.set_uint("css-last-update", this.cssTag);
+          return true;
+        }
+      } catch (e) {
+        console.warn(`Forge: can't update the stylesheet ${configCss.get_path()}: ${e.message}`);
       }
     }
     return false;
   }
 
+  /**
+   * Copies `source` over `target`. The copy gets the usual permissions of a new file, not those
+   * of `source`: the extension's own files can be read-only (e.g. on NixOS), and a read-only copy
+   * couldn't be overwritten at the next update (#266). A read-only copy left by an earlier
+   * version is replaced.
+   * @param {Gio.File} source
+   * @param {Gio.File} target
+   */
+  _copyFile(source, target) {
+    const flags = Gio.FileCopyFlags.OVERWRITE | Gio.FileCopyFlags.TARGET_DEFAULT_PERMS;
+    try {
+      return source.copy(target, flags, null, null);
+    } catch (e) {
+      // (only a target that exists and is read-only: not when the source can't be read)
+      const readOnlyTarget =
+        e.matches(Gio.IOErrorEnum, Gio.IOErrorEnum.PERMISSION_DENIED) &&
+        target.query_exists(null) &&
+        !target
+          .query_info("access::can-write", Gio.FileQueryInfoFlags.NONE, null)
+          .get_attribute_boolean("access::can-write");
+      if (!readOnlyTarget) throw e;
+      target.delete(null);
+      return source.copy(target, flags, null, null);
+    }
+  }
+
   /**
    * Credits: ExtensionSystem.js:_callExtensionEnable()
    */