Add new command 'toggle-theme'

* doc/emacs/custom.texi (Custom Themes): Mention it.
* etc/themes/leuven-dark-theme.el (leuven-dark): Add dual theme.
* etc/themes/leuven-theme.el (leuven): Add dual theme.
* etc/themes/tango-dark-theme.el (tango-dark): Add dual theme.
* etc/themes/tango-theme.el (tango): Add dual theme.
* etc/themes/tsdh-dark-theme.el (tsdh-dark): Add dual theme.
* etc/themes/tsdh-light-theme.el (tsdh-light): Add dual theme.
* lisp/cus-theme.el (describe-theme-1): Say if a theme has a dual.
* lisp/custom.el (toggle-theme): Add new command.

9 files changed, 40 insertions, 2 deletions

diff --git a/doc/emacs/custom.texi b/doc/emacs/custom.texi
index ff7ab83190c..3dabba9d2fd 100644
--- a/doc/emacs/custom.texi
+++ b/doc/emacs/custom.texi
@@ -667,6 +667,11 @@ type @kbd{M-x disable-theme}.
 the @file{*Custom Themes*} buffer; or type @kbd{M-x describe-theme}
 anywhere in Emacs and enter the theme name.
 
+@findex toggle-theme
+  Some themes have dual variants (most often these are light and dark
+pairs).  You can switch between these by typing @kbd{M-x
+toggle-theme}.  Note that this only works if only one theme is active.
+
 @node Creating Custom Themes
 @subsection Creating Custom Themes
 @cindex custom themes, creating
diff --git a/etc/themes/leuven-dark-theme.el b/etc/themes/leuven-dark-theme.el
index 0e162c8bab9..cef40782c1b 100644
--- a/etc/themes/leuven-dark-theme.el
+++ b/etc/themes/leuven-dark-theme.el
@@ -5,7 +5,7 @@
 ;; Author: Fabrice Niessen <(concat "fniessen" at-sign "pirilampo.org")>
 ;; Contributor: Thibault Polge <(concat "thibault" at-sign "thb.lt")>
 ;; URL: https://github.com/fniessen/emacs-leuven-dark-theme
-;; Version: 20220202.1126
+;; Version: 20220906.2016
 ;; Keywords: color theme
 
 ;; This file is part of GNU Emacs.
@@ -1083,6 +1083,8 @@ more...")
   (add-to-list 'custom-theme-load-path
                (file-name-as-directory (file-name-directory load-file-name))))
 
+(put 'leuven-dark 'dual-theme 'leuven) ;see `toggle-theme'
+
 (provide-theme 'leuven-dark)
 
 ;; This is for the sake of Emacs.
diff --git a/etc/themes/leuven-theme.el b/etc/themes/leuven-theme.el
index d9a8d5391ae..73ac96d28d9 100644
--- a/etc/themes/leuven-theme.el
+++ b/etc/themes/leuven-theme.el
@@ -4,7 +4,7 @@
 
 ;; Author: Fabrice Niessen <(concat "fniessen" at-sign "pirilampo.org")>
 ;; URL: https://github.com/fniessen/emacs-leuven-theme
-;; Version: 20200513.1928
+;; Version: 20220906.2016
 ;; Keywords: color theme
 
 ;; This file is part of GNU Emacs.
@@ -1065,6 +1065,8 @@ more...")
   (add-to-list 'custom-theme-load-path
                (file-name-as-directory (file-name-directory load-file-name))))
 
+(put 'leuven 'dual-theme 'leuven-dark) ;see `toggle-theme'
+
 (provide-theme 'leuven)
 
 ;; This is for the sake of Emacs.
diff --git a/etc/themes/tango-dark-theme.el b/etc/themes/tango-dark-theme.el
index ef00d2ac49f..f9f2692ac59 100644
--- a/etc/themes/tango-dark-theme.el
+++ b/etc/themes/tango-dark-theme.el
@@ -190,6 +190,8 @@ Semantic, and Ansi-Color faces are included.")
    `(ansi-color-bright-white ((,class (:background ,alum-1
 				       :foreground ,alum-1))))))
 
+(put 'tango-dark 'dual-theme 'tango) ;see `toggle-theme'
+
 (provide-theme 'tango-dark)
 
 ;;; tango-dark-theme.el ends here
diff --git a/etc/themes/tango-theme.el b/etc/themes/tango-theme.el
index ecbbf037536..8d1686f05e1 100644
--- a/etc/themes/tango-theme.el
+++ b/etc/themes/tango-theme.el
@@ -173,6 +173,8 @@ Semantic, and Ansi-Color faces are included.")
    `(ansi-color-bright-white ((,class (:background ,alum-1
 				       :foreground ,alum-1))))))
 
+(put 'tango 'dual-theme 'tango-dark) ;see `toggle-theme'
+
 (provide-theme 'tango)
 
 ;;; tango-theme.el ends here
diff --git a/etc/themes/tsdh-dark-theme.el b/etc/themes/tsdh-dark-theme.el
index a88ad75520b..ad54eb3df01 100644
--- a/etc/themes/tsdh-dark-theme.el
+++ b/etc/themes/tsdh-dark-theme.el
@@ -142,6 +142,8 @@
  '(widget-field ((t (:box (:line-width 2 :color "grey75" :style pressed-button)))))
  '(window-number-face ((t (:foreground "red" :weight bold)))))
 
+(put 'tsdh-dark 'dual-theme 'tsdh-light) ;see `toggle-theme'
+
 (provide-theme 'tsdh-dark)
 
 ;;; tsdh-dark-theme.el ends here
diff --git a/etc/themes/tsdh-light-theme.el b/etc/themes/tsdh-light-theme.el
index d9d09b702b7..64c048f7e5e 100644
--- a/etc/themes/tsdh-light-theme.el
+++ b/etc/themes/tsdh-light-theme.el
@@ -104,6 +104,8 @@ Used and created by Tassilo Horn.")
  '(show-paren-mismatch ((t (:background "deep pink" :weight bold))))
  '(window-number-face ((t (:foreground "red" :weight bold)))))
 
+(put 'tsdh-light 'dual-theme 'tsdh-dark) ;see `toggle-theme'
+
 (provide-theme 'tsdh-light)
 
 ;;; tsdh-light-theme.el ends here
diff --git a/lisp/cus-theme.el b/lisp/cus-theme.el
index 90680ff68f8..80cae6d663c 100644
--- a/lisp/cus-theme.el
+++ b/lisp/cus-theme.el
@@ -515,6 +515,8 @@ It includes all faces in list FACES."
 			  (end-of-file nil)))))
             (and (eq (car-safe sexp) 'deftheme)
 		 (setq doc (nth 2 sexp)))))))
+    (when-let ((dual (get theme 'dual-theme)))
+      (princ (format " The dual theme is `%s'" dual)))
     (princ "\n\nDocumentation:\n")
     (princ (if (stringp doc)
 	       (substitute-command-keys doc)
diff --git a/lisp/custom.el b/lisp/custom.el
index 352b5b0e160..b4d1ba7317f 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -1372,6 +1372,25 @@ Return t if THEME was successfully loaded, nil otherwise."
     (enable-theme theme))
   t)
 
+(defun toggle-theme (&optional no-confirm no-enable)
+  "Toggle the current active theme by enabling its dual pair.
+The current theme will be immediately disabled before the dual
+theme has been enabled.  If THEME is not active an error will be
+raised.  If theme is nil For NO-CONFIRM and NO-ENABLE, see
+`load-theme'."
+  (interactive)
+  (cond
+   ((length= custom-enabled-themes 0)
+    (user-error "No theme is active, cannot toggle"))
+   ((length> custom-enabled-themes 1)
+    (user-error "More than one theme active, cannot unambiguously toggle")))
+  (let* ((theme (car custom-enabled-themes))
+         (dual (get theme 'dual-theme)))
+    (unless dual
+      (error "`%s' has no dual theme to toggle between" theme))
+    (disable-theme theme)
+    (load-theme dual no-confirm no-enable)))
+
 (defun custom-theme-load-confirm (hash)
   "Query the user about loading a Custom theme that may not be safe.
 The theme should be in the current buffer.  If the user agrees,