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

[Alberto Fanjul <albfan src gnome org>]

commit a8112c1989f00fc8738eeb21c39c646117825549
Author: Alberto Fanjul <albertofanjul gmail com>
Date:   Fri Jun 29 08:16:32 2018 +0200

    theme: Detect dark theme with its background color

 libgitg/gitg-diff-view-file-renderer-text.vala | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)
---
diff --git a/libgitg/gitg-diff-view-file-renderer-text.vala b/libgitg/gitg-diff-view-file-renderer-text.vala
index ee7b2b56..d83e9f19 100644
--- a/libgitg/gitg-diff-view-file-renderer-text.vala
+++ b/libgitg/gitg-diff-view-file-renderer-text.vala
@@ -550,11 +550,22 @@ class Gitg.DiffViewFileRendererText : Gtk.SourceView, DiffSelectable, DiffViewFi
 
                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)
                {
-                       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 };
-                       removed_attributes.background = Gdk.RGBA() { red = 164.0 / 255.0, green = 0.0, blue = 
0.0, alpha = 1.0 };
+                       header_attributes.background = Gdk.RGBA() { red = 88.0 / 255.0, green = 88.0 / 255.0, 
blue = 88.0 / 255.0, alpha = 1.0 };
+                       added_attributes.background = Gdk.RGBA() { red = 32.0 / 255.0, green = 68.0 / 255.0, 
blue = 21.0 / 255.0, alpha = 1.0 };
+                       removed_attributes.background = Gdk.RGBA() { red = 130.0 / 255.0, green = 55.0 / 
255.0, blue = 53.0 / 255.0, alpha = 1.0 };
                }
                else
                {