The missing css file error is intentional - we include the file in the templates by default but do not supply a source file for it - it is for custom user css overrides if you would like to change the look/feel of the webapp without actually editing templates you can provide your own css.
From outside your LAN? Make sure that your ISP doesn't filter those ports you are using, otherwise just use a different port and map it to the internal port (80 or 443) and it should work fine.
You can bypass nginx all together by going straight to port 5000 for what it's worth, nginx just provides a lot of things gunicorn itself doesn't.