Support dark mode

Follows the system preference for dark mode.

See https://flatpak.github.io/xdg-desktop-portal/#gdbus-org.freedesktop.portal.Settings.

meson.build

@@ -54,6 +54,7 @@ if enable_runtime_dependency_checks
     ]
 
     dependency('pygobject-3.0', required: true)
+    dependency('libhandy-1', version: '>= 1.6')
 else
     # external python modules that are required for building piper
     python_modules = ['lxml']

piper/application.py

@@ -6,7 +6,8 @@
 import gi
 gi.require_version("Gio", "2.0")
 gi.require_version("Gtk", "3.0")
-from gi.repository import Gio, GLib, Gtk  # noqa
+gi.require_version("Handy", "1")
+from gi.repository import Gio, GLib, Gtk, Handy  # noqa
 
 
 class Application(Gtk.Application):
@@ -27,9 +28,14 @@ def do_startup(self):
         initialization should be done here, to prevent doing duplicate work in
         case another window is opened."""
         Gtk.Application.do_startup(self)
+        Handy.init()
         self._build_app_menu()
         self._ratbagd = None
 
+        # Opt-in to follow the system color-scheme.
+        manager = Handy.StyleManager.get_default()
+        manager.set_color_scheme(Handy.ColorScheme.PREFER_LIGHT)
+
     def init_ratbagd(self):
         if self._ratbagd is None:
             self._ratbag = Ratbagd(self._required_ratbagd_version)