Add offline warning site

[lutzseverino]

I have recreated the "Computer not connected" site to display a warning when the host's computer is unable to connect to the internet. There are multiple ways to implement this feature, so I didn't do it myself, I'll leave the choice the maintainer.

1. Colors are consistent with the rest of the application.
2. CSS is set up to follow the user's theme.

Dark mode

Light mode