Add new command 'copy-theme-options'

* doc/emacs/custom.texi (Custom Themes): Mention command in a
general context.
(Newcomers Theme): Mention command in the context of the
'newcomers-preset' theme
* etc/NEWS: Mention new command.
* lisp/custom.el (copy-theme-options): New command.

doc/emacs/custom.texi

@@ -656,12 +656,17 @@ theme, its @samp{State} display shows @samp{THEMED} instead of
 @findex load-theme
 @findex enable-theme
 @findex disable-theme
+@findex copy-theme-options
   You can enable a specific Custom theme in the current Emacs session
 by typing @kbd{M-x load-theme}.  This prompts for a theme name, loads
 the theme from the theme file, and enables it.  If a theme file
 has been loaded before, you can enable the theme without loading its
 file by typing @kbd{M-x enable-theme}.  To disable a Custom theme,
-type @kbd{M-x disable-theme}.
+type @kbd{M-x disable-theme}.  Some themes provide a set of user options
+(@pxref{Newcomers Theme}), that might vary over time.  If you want to
+persist the current set of options, you can copy the user option of a
+theme into your own configuration, you can use the
+@code{copy-theme-options} command.
 
 @findex describe-theme
   To see a description of a Custom theme, type @kbd{?} on its line in
@@ -3249,4 +3254,6 @@ conventions from other programs trying to learn Emacs.
   Keep in mind that the functionality enabled by this theme may change
 between releases, including adding new functionality that is expected to
 be of value for new users or removing old functionality that is regarded
-to be confusing or superseded.
+to be confusing or superseded.  You can use a command like
+@code{copy-theme-options} to copy the options from
+@code{newcomers-presets} directly into your own configuration.

etc/NEWS

@@ -865,6 +865,11 @@ on graphical displays then when this option is enabled Emacs will use a
 purple bar cursor on compatible terminals as well.  See the Info node
 "(emacs) Cursor Display" for more information.
 
+---
+** New command 'copy-theme-options'
+You can use this command to copy options from a theme into your user
+configuration.
+
 
 * Editing Changes in Emacs 31.1
 

lisp/custom.el

@@ -1821,6 +1821,33 @@ If a choice with the same tag already exists, no action is taken."
       (push load loads)))
   (put symbol 'custom-loads loads))
 
+;;;###autoload
+(defun copy-theme-options (theme)
+  "Copy settings from THEME into the user theme."
+  (interactive (list (intern (completing-read "Copy from theme: "
+                                              (custom-available-themes)
+                                              nil t))))
+  (load-theme theme nil t)
+  (when (null (get theme 'theme-settings))
+    (message "Theme `%s' has no user options" theme))
+  (pcase-dolist (`(theme-value ,var ,(pred (eq theme _)) ,val)
+                 (get theme 'theme-settings))
+    ;; If the proposed value is already the current value, then we
+    ;; ignore the value from the theme and don't overwrite any comments.
+    ;; Otherwise we check if the current value is the default value, and
+    ;; if not we prompt the user if they are OK with overwriting their
+    ;; previous configuration.
+    (when (let ((val* (funcall (or (get var 'custom-get) #'default-value) var))
+                (val (eval (with-demoted-errors "%S" val) t)))
+            (and (not (equal val val*))
+                 (or (custom--standard-value-p var val*)
+                     (yes-or-no-p (format "You have customized `%s' to %S, \
+overwrite with %S?"
+                                          var (symbol-value var) val)))))
+      (customize-save-variable
+       var val
+       (format "Copied from %s by `copy-theme-options'" theme)))))
+
 (provide 'custom)
 
 ;;; custom.el ends here