Newer versions of Org mode change the default face for DONE headlines. The Spacemacs theme should probably provide a sane default face in this case. See also https://github.com/syl20bnr/spacemacs/issues/15055
Hello @dschrempf thanks for reporting this. I've added two new faces for org todo headlines, and I've turned the org-fontify-done-headline off by default.
Newer versions of Org mode change the default face for DONE headlines. The Spacemacs theme should probably provide a sane default face in this case. See also https://github.com/syl20bnr/spacemacs/issues/15055