bug#48351: 28.0.50; custom-theme-summary doesn't find theme summary line reliably

[Mauro Aranda <maurooaranda@gmail.com>]

Starting from emacs -Q:
M-x customize-themes

Look for themes such as the leuven theme or the modus-* themes.  The
summary after the checkbox reads: "(no documentation available)"

However looking at the theme files, the deftheme form has a docstring,
so I think the *Custom Themes* buffer should be showing the summary
correctly.

IMO, the restriction imposed by `custom-theme-summary' is too strict:
the function currently requires that the deftheme form is the very first
one in the file.


In GNU Emacs 28.0.50 (build 2, i686-pc-linux-gnu, GTK+ Version 2.24.33, cairo version 1.16.0)
 of 2021-05-11 built on tbb
Repository revision: cd46972ee82d392cef8fa6e51c370d3d8f48777a
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Slackware 14.2 i586 (post 14.2 -current)

Configured using:
 'configure --with-mailutils
 PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/pkgconfig:/usr/share/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND
THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK2 ZLIB

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  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
rfc822 mml mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map 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 cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
iso-transl tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd 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 easymenu timer select scroll-bar mouse jit-lock
font-lock syntax 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 button loaddefs faces cus-face macroexp files
window text-properties overlay sha1 md5 base64 format env code-pages
mule custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 8 50866 5885)
 (symbols 24 6591 1)
 (strings 16 18158 1687)
 (string-bytes 1 599925)
 (vectors 8 13197)
 (vector-slots 4 179810 14964)
 (floats 8 21 47)
 (intervals 28 197 4)
 (buffers 564 10))