[gimp] app, libgimp: communicate dark-theme preference to plug-ins through theme.css

[Ell <ell src gnome org>]

commit 8c96c3d1bbb29fd181328dd6ca6afc3ade1b2257
Author: Ell <ell_se yahoo com>
Date:   Wed Feb 20 14:27:59 2019 -0500

    app, libgimp: communicate dark-theme preference to plug-ins through theme.css
    
    The GUI implementation of gimp_wait() relies on the ability to run
    plug-ins (namely, the busy-dialog plug-in) without entering the
    main loop.  This prohibits the said plug-ins from making any PDB
    calls, which would result in a deadlock.  However, we're currently
    calling gimp_gimprc_query() to fetch the prefer-dark-theme option
    during gimp_ui_init() (or any time the theme.css file changes).
    
    Instead, communicate this preference through the theme.css file
    itself, by writing a /* prefer-dark-theme */ comment to the file
    when the option is set.  Yes, it's a bit of a hack :P

 app/gui/themes.c |  4 +++-
 libgimp/gimpui.c | 28 ++++++++++++++++++++--------
 2 files changed, 23 insertions(+), 9 deletions(-)
---
diff --git a/app/gui/themes.c b/app/gui/themes.c
index c79a4e275d..f022c7ac3d 100644
--- a/app/gui/themes.c
+++ b/app/gui/themes.c
@@ -285,11 +285,13 @@ themes_apply_theme (Gimp          *gimp,
              "\n"
              "* { -gtk-icon-style: %s; }\n"
              "\n"
+             "%s"
              "/* end of theme.css */\n",
              gimp_file_get_utf8_name (css_user),
              esc_css_theme,
              esc_css_user,
-             config->prefer_symbolic_icons ? "symbolic" : "regular"))
+             config->prefer_symbolic_icons ? "symbolic" : "regular",
+             config->prefer_dark_theme ? "/* prefer-dark-theme */\n\n" : ""))
         {
           GCancellable *cancellable = g_cancellable_new ();
 
diff --git a/libgimp/gimpui.c b/libgimp/gimpui.c
index 2083dddd47..018cecc84a 100644
--- a/libgimp/gimpui.c
+++ b/libgimp/gimpui.c
@@ -349,17 +349,29 @@ gimp_ui_theme_changed (GFileMonitor      *monitor,
                        GFileMonitorEvent  event_type,
                        GtkCssProvider    *css_provider)
 {
-  gchar    *dark    = gimp_gimprc_query ("prefer-dark-theme");
-  gboolean  setting = dark && ! strcmp (dark, "yes");
-  GError   *error   = NULL;
+  GError *error = NULL;
+  gchar  *contents;
 
-  g_object_set (gtk_settings_get_for_screen (gdk_screen_get_default ()),
-                "gtk-application-prefer-dark-theme", setting,
-                NULL);
+  file = gimp_directory_file ("theme.css", NULL);
 
-  g_free (dark);
+  if (g_file_load_contents (file, NULL, &contents, NULL, NULL, &error))
+    {
+      gboolean prefer_dark_theme;
 
-  file = gimp_directory_file ("theme.css", NULL);
+      prefer_dark_theme = strstr (contents, "/* prefer-dark-theme */") != NULL;
+
+      g_object_set (gtk_settings_get_for_screen (gdk_screen_get_default ()),
+                    "gtk-application-prefer-dark-theme", prefer_dark_theme,
+                    NULL);
+
+      g_free (contents);
+    }
+  else
+    {
+      g_printerr ("%s: error loading %s: %s\n", G_STRFUNC,
+                  gimp_file_get_utf8_name (file), error->message);
+      g_clear_error (&error);
+    }
 
   if (! gtk_css_provider_load_from_file (css_provider, file, &error))
     {