Emacs face definition forms are either
((DISPLAY . PLIST)
(DISPLAY . PLIST))
or
((DISPLAY PLIST) ;For backward compatibility.
(DISPLAY PLIST))
Commit a2388bc56e (2020-08-08) follows
neither of the correct formats. It defines:
`((((class color) (background light))
,@(and (>= emacs-major-version 27) '(:extend t))
(:background "#f0f0f0"))
(((class color) (background dark))
,@(and (>= emacs-major-version 27) '(:extend t))
(:background "#303030")))
which produces:
((DISPLAY
:extend t (:background "#f0f0f0"))
(DISPLAY
:extend t (:background "#303030")))
And that is wrong format.
This change fixes the face definition form to produce:
((DISPLAY
:extend t :background "#f0f0f0")
(DISPLAY
:extend t :background "#303030"))
which follows the (DISPLAY . PLIST) format (see above).