HTML body has no background color

The background color is missing, which causes text readability issues on a dark background. This bug is in addition to another similar bug in kiwix-serve. Both bugs were mentioned in this thread. https://github.com/kiwix/kiwix-tools/issues/742#issuecomment-2891316405

![Image](https://github.com/user-attachments/assets/d64a1bd0-9ea9-4c6a-997f-ffb77e4f2672)