Most icons are invisible on freecodecamp forum, however the mouseover highlights the button and displays text. It is likely blocked by the firewall. Adding text/color to the buttons makes them visible. Is this a common problem?
Can you provide a screenshot of what you are referring to when you say “invisible”? Maybe highlight where the icon appears invisible to you.
I think the bootstrap icons source may be blocked by the firewall, but I can see the text that is next to the icon, the color change grey mouse over, and the text that appears when mouse over. It does make them somewhat hard to find though.