Change header logo link?

You're welcome. Hm, afaik theme-lib stuff is basically customizations that professional services "install" when they do customer work. But I have seen it mostly as an imported macro like theme-lib.common-functions or something like that. But if they are custom components you could also just ask support to make sure you can see them in Studio.

On the other hand, the specific component you are talking about is so simple I wouldn't even bother modifying the original one (which you could by @override'ing it) but simply make your own with those few lines of HTML and then replace it in the quilt.

If you know (or are worried) that you would have to change many quilts to replace such a global component, the @override approach is what I would go for, you'd basically create a new component in studio with the same name and that suffix, e.g. 

theme-lib.community-header-logo@override 

then you just paste in your own changed HTML and you are good to go, no need to replace anything in any quilt.

Technically if you wanted to also access or use the original component's output somehow you can use the <@delegate /> directive of FreeMarker to get that within an overridden component. But in your case I don't think that is needed.