Standard value of defcustom doesn't match with type
To reproduce:
emacs -Q
(load-theme 'modus-operandi 'no-confirm 'no-enable)
M-x customize-option RET modus-themes-headings
Custom reports a mismatch, because the standard value:
((t . nil))
doesn't match the defcustom type.
More specifically, nil is not part of the choices allowed.