I can confirm this was happening on my FF install as well.
I believe this is a side effect of the "Privacy Shield" -- I disabled it and things appear to render correctly.
(Now nothing is loading in search results, but that out of scope for this discussion)
EDIT: Actually, I must've just refreshed when a change went out, or otherwise triggered a cache clear with the privacy shield. The UI works regardless of its setting now -- which makes sense, it never "detected" any trackers, and Kagi doesn't use anything that I'd imagine would hit that filter.