Change the emacs theme to light or dark variant (GTK+-3)

init.el

(defun set-emacs-frames (variant)
  (dolist (frame (frame-list))
    (let* ((window-id (frame-parameter frame 'outer-window-id))
	   (id (string-to-number window-id))
	   (cmd (format "xprop -id 0x%x -f _GTK_THEME_VARIANT 8u -set _GTK_THEME_VARIANT \"%s\""
			id variant)))
      (call-process-shell-command cmd))))

(defun set-emacs-theme-light ()
  (interactive)
  (load-theme 'solarized-light t)
  (set-emacs-frames "light"))

(defun set-emacs-theme-dark ()
  (interactive)
  (load-theme 'solarized-dark t)
  (set-emacs-frames "dark"))

(if (window-system)
    (set-emacs-theme-light))

Is this going to work for Wayland?

In its current version, unlikely. It uses xprop, which is X11 tool. Also the window-id part might not work either. But perhaps it won't be too hard to fix.

I'm still not running Wayland so I haven't tried it.