base URL for external stylesheet ?

Depending on webhost provider, a subdomain may point to your domain.com/demo folder, or it can be a completely different folder. Depending on how you can set-up things.

example.com
/public_html

demo.example.com
/demo.example.com/public

If you can do the second option, specifying your own folder. Point it into the public folder and place your assets folder alongside your index.php. So that you can only specify "url('/assets/img/arrow-top.svg');"