Allow toggling between dark and light GTK themes (if provided).

Took me a bit to get the icon fallbacks working right.

28min.

data/meson.build

@@ -2,7 +2,7 @@ install_data('icons/write-as.png',
     install_dir: join_paths(get_option('datadir'), 'icons', 'hicolor', '128x128', 'apps')
 )
 install_data('icons/writeas-bright-dark.png',
-    install_dir: join_paths(get_option('datadir'), 'icons', 'hicolor', '48x48', 'actions'))
+    install_dir: join_paths(get_option('datadir'), 'icons', 'hicolor', '16x16', 'actions'))
 
 install_data('write-as-gtk.desktop',
     install_dir: join_paths(get_option('datadir'), 'applications'))

src/window.vala

@@ -2,10 +2,7 @@ public class WriteAs.MainWindow : Gtk.ApplicationWindow {
     private Gtk.TextView canvas;
 
     construct {
-        var header = new Gtk.HeaderBar();
-        header.title = "";
-        header.show_close_button = true;
-        set_titlebar(header);
+        construct_toolbar();
 
         canvas = new Gtk.TextView();
         add(canvas);
@@ -16,4 +13,25 @@ public class WriteAs.MainWindow : Gtk.ApplicationWindow {
 
         set_default_size(800, 600);
     }
+
+    private void construct_toolbar() {
+        var header = new Gtk.HeaderBar();
+        header.title = "";
+        header.show_close_button = true;
+        set_titlebar(header);
+
+        var darkmode_button = new Gtk.ToggleButton();
+        // NOTE the fallback icon is a bit of a meaning stretch, but it works.
+        var icon_theme = Gtk.IconTheme.get_default();
+        darkmode_button.image = new Gtk.Image.from_icon_name(
+                icon_theme.has_icon("writeas-bright-dark") ?
+                    "writeas-bright-dark" : "weather-clear-night",
+                Gtk.IconSize.SMALL_TOOLBAR);
+        darkmode_button.draw_indicator = false;
+        var settings = Gtk.Settings.get_default();
+        darkmode_button.toggled.connect(() => {
+            settings.gtk_application_prefer_dark_theme = darkmode_button.active;
+        });
+        header.pack_end(darkmode_button);
+    }
 }