[gnome-builder] app: set GtkSettings:gtk-theme-name under Flatpak

[Christian Hergert <chergert src gnome org>]

commit b72fceaa7873778b1a43cabfc49ffbf37b0299dc
Author: Christian Hergert <chergert redhat com>
Date:   Fri Feb 24 18:09:14 2017 -0800

    app: set GtkSettings:gtk-theme-name under Flatpak
    
    In general, we only support Adwaita. But we definitely want to stick to
    that if we are inside of a Flatpak so that we can put our best foot
    forward, style-wise.

 libide/application/ide-application.c |   21 +++++++++++++++++++--
 1 files changed, 19 insertions(+), 2 deletions(-)
---
diff --git a/libide/application/ide-application.c b/libide/application/ide-application.c
index 6be3577..07d8dc7 100644
--- a/libide/application/ide-application.c
+++ b/libide/application/ide-application.c
@@ -44,6 +44,7 @@
 #include "resources/ide-resources.h"
 #include "theming/ide-css-provider.h"
 #include "theming/ide-theme-manager.h"
+#include "util/ide-flatpak.h"
 #include "workbench/ide-workbench.h"
 #include "workers/ide-worker.h"
 
@@ -102,13 +103,29 @@ ide_application_register_theme_overrides (IdeApplication *self)
 
   g_assert (IDE_IS_APPLICATION (self));
 
-  self->theme_manager = ide_theme_manager_new ();
-
   screen = gdk_screen_get_default ();
   gtk_settings = gtk_settings_get_for_screen (screen);
   settings = g_settings_new ("org.gnome.builder");
 
   /*
+   * As early as possible, overwrite the gtk theme if we are running in
+   * flatpak. We want to ensure that we provide the best visual appearance
+   * that we can, for which we only support the internal Gtk theme currently.
+   *
+   * If we can get a designer that manages other themes and keeps them up
+   * to date and working inside flatpak, we can consider doing something
+   * different here.
+   */
+  if (ide_is_flatpak () && g_getenv ("GTK_THEME") == NULL)
+    {
+      g_object_set (gtk_settings,
+                    "gtk-theme-name", "Adwaita",
+                    NULL);
+    }
+
+  self->theme_manager = ide_theme_manager_new ();
+
+  /*
    * Some users override the "default to dark theme" in gnome-tweak-tool,
    * which means if they haven't selected the dark theme, we will
    * inadvertantly set the application to light-mode. If we detect this,