It would be nice if hl-fill-colum-face would behave the same as hl-line. I've tried (add-to-list 'solaire-mode-remap-alist '((hl-fill-column-face solaire-hl-line-face) . t)) to no avail.
This was addressed in hlissner/emacs-doom-themes, but 2be8aa0 also introduces a change in how solaire-mode works, which should indirectly address this issue. Let me know if that isn't the case and I'll reopen it. Thanks for bringing it to my attention!