Hello

.registered_user::after {
content: " \00AE";
}

What, do I delete this code? And from what location?

Search for ".registered_user::after" in the file "style.min.css" of the active theme (in the default theme it's the line #208 in style.min.css) and remove the shown code including the content of {}. BUT remember the fact, that every update of the forum script could remove your change. Changes should be seldom but they are possible.

So maybe it's a good idea to copy the default theme into a new one, make the changes there and select the new theme in the admin area after it's upload with a new name. If the structure of the code and with it the standard theme changes, it's fairly possible that your theme has to be altered too. But – that's important to note – your initial changes are not overwritten/lost at that point.

Tschö, Auge