However, the css file is loaded by directly calling .css, which using Kotomi means that it's looking for the css page within the current folder. I addressed that by adding a Template Modification that references the boardUrl and thus pretty much "hard-code" it to grab the right file.
Now I'm having a minor issue where the non-default XenForo CSS is not properly loaded, and I'm hoping that I can somehow pull the same trick for that. I sidestepped the issue for now by hard-coding the call to the css I can see in the page's source, but obviously this is far from optimal (adding a new add-on would mean the line would have to be fixed again).
In PAGE_CONTAINER, the following line appears: