Style Sheets with Insecure URL(s)

First off, I'm a noob.  Probably worse than a noob and I really hope I'm not intruding.

I'm using Win 10 pro and running the latest version of Xampp.  As an aside I'm really liking Xampp.

I have a domain set up via DuckDNS and it works, got the DuckDNS updater on my machine, running without issue.  Have a Static IP on the server machine, port 80 and 443 forwarded.  The website pings via command line.

And then so I have configured public SSL access.  Real deal.  I got a 90 day certificate and private key from ZeroSSL and according to MS edge my certificate is installed correctly and is valid.

I can access the Elgg login landing page from localhost and from http:// on the server machine.

From all machines, server and otherwise attempting to access the Elgg landing page via https:// I'm getting the simple, left justified blue text screen and I have been attempting to correct this problem for over a week now.

Then I blundered across a website for troubleshooting https:// access: "Why No Padlock?"  I really have to tell you, this website looks to me to be totally thorough.  (But then what do I know?)  Unfortunately the countermeasures proposed appear to me, the noob, as being written in Martian. For instance: "A style-sheet with an insecure url of "http://redrubberball/cache/1611439564/default/font-awesome/css/all.min.css" was loaded on line: 2 of https://xxxxxxxxx.duckdns.org/."

I can give you the web address if you need it to troubleshoot this problem.  Can someone please tell me how to correct these problem(s)?  Please type slowly because I don't read too fast and thank you in advance for your assistance in correcting this problem and very best regards.