Automatic synchronization between Emacs themes and system appearance settings (light/dark mode) via D-Bus.

appearance.el

;;; appearance.el --- System theme integration via D-Bus -*- lexical-binding: t -*-

;;; Commentary:
;; 
;; This module provides automatic synchronization between Emacs themes
;; and the system appearance settings (light/dark mode) via D-Bus.
;;
;; NOTE: This *only* works with desktop environments that support the
;; org.freedesktop.appearance portal interface.

;;; Code:

(require 'dbus)

(defgroup appearance nil
  "System appearance synchronization for Emacs."
  :group 'faces
  :prefix "appearance-")

(defcustom appearance-light-theme 'modus-operandi-tinted
  "Theme to use when system is using light mode."
  :type 'symbol
  :group 'appearance)

(defcustom appearance-dark-theme 'modus-vivendi-tinted
  "Theme to use when system is using dark mode."
  :type 'symbol
  :group 'appearance)

(defconst appearance--color-scheme-light 2
  "Value representing light color scheme from freedesktop portal.")

(defconst appearance--color-scheme-dark 1
  "Value representing dark color scheme from freedesktop portal.")

(defconst appearance--color-scheme-default 0
  "Value representing default (usually light) color scheme from freedesktop portal.")

(defvar appearance--current-theme nil
  "Currently active theme set by appearance module.")

;;;###autoload
(defun appearance-parse-color-scheme (value)
  "Apply appropriate theme based on system color scheme VALUE.
VALUE is an integer as defined by the freedesktop.org appearance portal:
- 0: Default (typically light)
- 1: Dark preference
- 2: Light preference"
  (let ((theme (cond
                ((or (= value appearance--color-scheme-light)
                     (= value appearance--color-scheme-default))
                 (prog1 appearance-light-theme
                   (set-frame-parameter nil 'background-mode 'light)))
                (t
                 (prog1 appearance-dark-theme
                   (set-frame-parameter nil 'background-mode 'dark))))))
    ;; Clean transition: unload previous theme only if different from target
    (when (and appearance--current-theme
               (not (eq appearance--current-theme theme))
               (custom-theme-enabled-p appearance--current-theme))
      (disable-theme appearance--current-theme))
    (unless (custom-theme-enabled-p theme)
      (load-theme theme t))
    (setq appearance--current-theme theme)
    ;; Ensure background-mode propagates to all frames
    (frame-set-background-mode nil)))

;;;###autoload
(defun appearance-setup-sync ()
  "Establish D-Bus connections for system appearance monitoring.
Creates two communication channels with the desktop portal:
1. Initial value retrieval to sync current system state
2. Signal registration for change notifications"
  (if (not (featurep 'dbusbind))
      (message "D-Bus support not available, cannot sync system appearance")
    (condition-case err
        (progn
          ;; Asynchronously retrieve current system preference to establish initial state
          (dbus-call-method-asynchronously
           :session
           "org.freedesktop.portal.Desktop"
           "/org/freedesktop/portal/desktop"
           "org.freedesktop.portal.Settings"
           "Read"
           (lambda (value)
             (when value
               (appearance-parse-color-scheme (car (car value)))))
           "org.freedesktop.appearance"
           "color-scheme")

          ;; Monitor for system preference changes in real-time
          (dbus-register-signal
           :session
           "org.freedesktop.portal.Desktop"
           "/org/freedesktop/portal/desktop"
           "org.freedesktop.portal.Settings"
           "SettingChanged"
           (lambda (namespace key variant)
             (when (and (string-equal namespace "org.freedesktop.appearance")
                        (string-equal key "color-scheme"))
               (let ((color-scheme (car variant)))
                 (when color-scheme
                   (appearance-parse-color-scheme color-scheme)))))))
      (error
       (message "Failed to set up appearance synchronization: %s" (error-message-string err))))))

;;;###autoload
(defun appearance-toggle-theme ()
  "Toggle between light and dark themes manually."
  (interactive)
  (if (eq appearance--current-theme appearance-light-theme)
      (appearance-parse-color-scheme appearance--color-scheme-dark)
    (appearance-parse-color-scheme appearance--color-scheme-light)))

;;;###autoload
(define-minor-mode appearance-sync-mode
  "Toggle automatic synchronization with system appearance.
When enabled, Emacs will switch between light and dark themes
based on the system appearance settings."
  :global t
  :init-value nil
  :lighter " AppSync"
  (if appearance-sync-mode
      (appearance-setup-sync)
    (when appearance--current-theme
      (message "Appearance sync disabled, theme remains %s" appearance--current-theme))))

(appearance-sync-mode 1)

(provide 'appearance)

;;; appearance.el ends here