[DOC] 404 page displays w/o CSS if address is a subfolder

[Simonbdy]

Insert the documentation link:
Every wrong address with a subfolder. Ex: https://docs.luos.io/wrong-subfolder/wrong-page.html

Describe the issue or the suggestion:
https://docs.luos.io/wrong-subfolder/wrong-page.html -> No CSS
https://docs.luos.io/wrong-page.html -> CSS ok

How would you fix the issue / which content would you add or delete?
It seems {{ path_to_root }} doesn't work with index.hbs when 404.md is called, if in a subfolder.