bug#46396: 27.1.90; Ediff's non-focused diff section, background too light in --reverse

[bug-gnu-emacs_at_gnu . org]

I created a file a.txt with foo\n\nbar\n\nbaz\n and a file b.txt with 
foo0\n\nbar\n\nbaz1\n.

I open these files with "emacs -Q --reverse a.txt b.txt", and run M-x 
ediff-buffers RET for the two corresponding buffers.

If I press n or p to traverse the diff sections, the diff section in focus is 
readable, but the unfocused sections have a white background, in other words, 
the contrast seems too low. The unfocused sections are fine without the 
--reverse option.

Aside: Similar to --reverse, I have been using (invert-face 'default) for a 
simple dark theme in emacs. I can get a similar result in terminal emacs with 
"(setq frame-background-mode 'dark) (mapc 'frame-set-background-mode 
(frame-list))". These approaches to a simple dark theme came up in discussions 
on #emacs.

I encountered the issue above, as I was using magit, with (invert-face 
'default), and pressed `e` on a commit, wanting to view the diff in the 
side-by-side view.

Thank you,
-Brady

Data from M-x report-emacs-bug RET below:

In GNU Emacs 27.1.90 (build 1, x86_64-apple-darwin19.6.0, NS appkit-1894.60 
Version 10.15.7 (Build 19H2))
of 2021-01-15 built on clay.local
Repository revision: 488204cdc64b6a130042ecc64d59c4538287b81d
Repository branch: emacs-27
Windowing system distributor 'Apple', version 10.3.1894
System Description:  Mac OS X 10.15.7

Recent messages:
Computing differences between a.txt and b.txt ...
Buffer A: Processing difference region 0 of 2
Buffer B: Processing difference region 0 of 2
Processing difference regions ... done
Refining difference region 1 ...
Refining difference region 2 ...
Quit this Ediff session? (y or n) y
Making completion list... [5 times]
completing-read-default: Command attempted to use minibuffer while in minibuffer
Making completion list...

Configured using:
'configure --with-rsvg'

Configured features:
RSVG GLIB NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS
MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Text

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils ediff
ediff-merg ediff-mult ediff-wind ediff-diff ediff-help ediff-init
cl-loaddefs cl-lib ediff-util tooltip eldoc electric uniquify ediff-hook
vc-hooks lisp-float-type mwheel term/ns-win ns-win ucs-normalize
mule-util term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads kqueue cocoa ns lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 51551 8621)
(symbols 48 6696 1)
(strings 32 17852 1647)
(string-bytes 1 613638)
(vectors 16 10753)
(vector-slots 8 134394 11440)
(floats 8 34 173)
(intervals 56 229 0)
(buffers 1000 17))