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.

1 files changed, 19 insertions, 0 deletions

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,