[gitg] theme: Detect dark theme with its background color

[Alberto Fanjul <albfan src gnome org>]

commit 04bdad03bfa8d1876db3ba412d3c40d77e78874a
Author: Alberto Fanjul <albertofanjul gmail com>
Date:   Fri Jul 20 19:50:06 2018 +0200

    theme: Detect dark theme with its background color

 gitg/gitg-window.vala       | 11 +++++++++++
 libgitg/gitg-diff-stat.vala | 11 +++++++++++
 2 files changed, 22 insertions(+)
---
diff --git a/gitg/gitg-window.vala b/gitg/gitg-window.vala
index 58904c74..c05dda7a 100644
--- a/gitg/gitg-window.vala
+++ b/gitg/gitg-window.vala
@@ -373,6 +373,17 @@ public class Window : Gtk.ApplicationWindow, GitgExt.Application, Initable
 
                var dark = settings.gtk_application_prefer_dark_theme || (theme != null && 
theme.has_suffix(":dark"));
 
+               if (!dark) {
+                       var stylecontext = get_style_context();
+                       Gdk.RGBA rgba;
+                       var background_set = stylecontext.lookup_color("theme_bg_color", out rgba);
+
+                       if (background_set && rgba.red + rgba.green + rgba.blue < 1.0)
+                       {
+                               dark = true;
+                       }
+               }
+
                if (dark)
                {
                        get_style_context().add_class("dark");
diff --git a/libgitg/gitg-diff-stat.vala b/libgitg/gitg-diff-stat.vala
index 0b4b986c..f83318fe 100644
--- a/libgitg/gitg-diff-stat.vala
+++ b/libgitg/gitg-diff-stat.vala
@@ -90,6 +90,17 @@ public class Gitg.DiffStat : Gtk.DrawingArea
 
                var dark = settings.gtk_application_prefer_dark_theme || (theme != null && 
theme.has_suffix(":dark"));
 
+               if (!dark) {
+                       var stylecontext = get_style_context();
+                       Gdk.RGBA rgba;
+                       var background_set = stylecontext.lookup_color("theme_bg_color", out rgba);
+
+                       if (background_set && rgba.red + rgba.green + rgba.blue < 1.0)
+                       {
+                               dark = true;
+                       }
+               }
+
                if (dark)
                {
                        get_style_context().add_class("dark");