Auto-detect system dark mode or light mode and select themes based on that

Question

Is there a way on linux (Ubuntu 20, Gnome) to detect changes in system's dark mode vs light mode and change emacs themes to a dark theme or light theme that I specify?

I see this question Change theme when OS dark mode changes? and several other resources on the internet which seem to work for MacOS only (there seems to be no ns-system-appearance-change-functions in my system when trying auto-complete with M-: as mentioned in the top answer)

Answer 1

As I don't use GNOME, I cannot say specifically with regards to that, but under KDE, it's pretty straight forward and presumably this can be adapted to GNOME with very few changes:

(when (and IS-LINUX ;; this is doom specific
           (featurep! :ui dbus)) ;; so is this
  ;; I should use a better name than `a`
  (defun theme--handle-dbus-event (a setting values)
    "Handler for FreeDesktop theme changes."
    (when (string= setting "ColorScheme")
      (let ((scheme (car values)))
        (cond
         ((string-match-p "Dark" scheme)
          (+theme-dark)) ;; my custom function that sets a dark theme
         ((string-match-p "Light" scheme)
          (+theme-light)) ;; 1000 internet points to whoever guesses what this does
         (t (message "I don't know how to handle scheme: %s" scheme))))))

  (require 'dbus)

  ;; since this is all FreeDesktop stuff, this *might* work on GNOME without changes
  (dbus-register-signal :session
                        "org.freedesktop.portal"
                        "/org/freedesktop/portal/desktop"
                        "org.freedesktop.impl.portal.Settings"
                        "SettingChanged"
                        #'theme--handle-dbus-event))