[gitg] Check also for GTK_THEME to determine dark theme

[Jesse van den Kieboom <jessevdk src gnome org>]

commit 377563a2adf2efeebbf5b8b9d8df6d4586c0039a
Author: Jesse van den Kieboom <jessevdk gnome org>
Date:   Mon Dec 21 22:33:54 2015 +0100

    Check also for GTK_THEME to determine dark theme

 libgitg/gitg-diff-view-file.vala |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/libgitg/gitg-diff-view-file.vala b/libgitg/gitg-diff-view-file.vala
index da13975..ccf360d 100644
--- a/libgitg/gitg-diff-view-file.vala
+++ b/libgitg/gitg-diff-view-file.vala
@@ -256,8 +256,11 @@ class Gitg.DiffViewFile : Gtk.Grid
                var removed_attributes = new Gtk.SourceMarkAttributes();
 
                var settings = Gtk.Settings.get_default();
+               var theme = Environment.get_variable("GTK_THEME");
 
-               if (settings.gtk_application_prefer_dark_theme)
+               var dark = settings.gtk_application_prefer_dark_theme || (theme != null && 
theme.has_suffix(":dark"));
+
+               if (dark)
                {
                        header_attributes.background = Gdk.RGBA() { red = 136.0 / 255.0, green = 138.0 / 
255.0, blue = 133.0 / 255.0, alpha = 1.0 };
                        added_attributes.background = Gdk.RGBA() { red = 78.0 / 255.0, green = 154.0 / 255.0, 
blue = 6.0 / 255.0, alpha = 1.0 };