I have hosted the CSS on a VPS and everything is working fine. But I am not able to connect a Solid App, like penny to this Server. Is https protocol mandatory for this connection?. Now, my server runs over http protocol. Any help would be appreciable.
I would expect so and would certainly recommend so, given that sensitive data will be sent over the wire. I know that at least Penny assumes HTTPS connections (which is not to say that it will definitely not work with HTTP connections, but those are untested and are not explicitly accounted for, so they probably won’t).
Yes, it should either run on https or for local testing on http://localhost. For instance, CSS denies web IDs with a http:// protocol and thus prevents login with an app, as it was the case in this issue: Access token trouble · CommunitySolidServer/CommunitySolidServer · Discussion #1603 · GitHub
Your issue may not even be related to Solid. Firefox and Chrome block active mixed content. I.e.
XHR requests over
https website won’t get sent in modern Chrome and Firefox.
So you need to send
https requests from
https web app, whether using Solid or not.