theme: Allow using custom colors from the GTK+ theme

Add an additional color type to pick up colors defined with
@define-color in the GTK+ theme's CSS:

  gtk:custom(name,fallback)

(where "name" refers to the name defined in GTK+'s CSS, and fallback
refers to an alternative color spec which is used when the color
referenced by "name" is not found)

The main intent of the change is to allow designers to improve
Adwaita's dark theme variant without having to compromise on colors
which work in the light variant as well.

https://bugzilla.gnome.org/show_bug.cgi?id=648709

NOTE: Patch copied from mutter and adapted for metacity.

3 files changed, 96 insertions, 2 deletions

diff --git a/src/ui/theme-parser.c b/src/ui/theme-parser.c
index b4186594..8f7c76fd 100644
--- a/src/ui/theme-parser.c
+++ b/src/ui/theme-parser.c
@@ -38,7 +38,7 @@
  * look out for.
  */
 #define THEME_MAJOR_VERSION 3
-#define THEME_MINOR_VERSION 3
+#define THEME_MINOR_VERSION 4
 #define THEME_VERSION (1000 * THEME_MAJOR_VERSION + THEME_MINOR_VERSION)
 
 #define MARCO_THEME_FILENAME_FORMAT "metacity-theme-%d.xml"
diff --git a/src/ui/theme.c b/src/ui/theme.c
index eb20285d..96dd25f0 100644
--- a/src/ui/theme.c
+++ b/src/ui/theme.c
@@ -1096,6 +1096,10 @@ meta_color_spec_new (MetaColorSpecType type)
       size += sizeof (dummy.data.gtk);
       break;
 
+    case META_COLOR_SPEC_GTK_CUSTOM:
+      size += sizeof (dummy.data.gtkcustom);
+      break;
+
     case META_COLOR_SPEC_BLEND:
       size += sizeof (dummy.data.blend);
       break;
@@ -1127,6 +1131,14 @@ meta_color_spec_free (MetaColorSpec *spec)
       DEBUG_FILL_STRUCT (&spec->data.gtk);
       break;
 
+    case META_COLOR_SPEC_GTK_CUSTOM:
+      if (spec->data.gtkcustom.color_name)
+        g_free (spec->data.gtkcustom.color_name);
+      if (spec->data.gtkcustom.fallback)
+        meta_color_spec_free (spec->data.gtkcustom.fallback);
+      DEBUG_FILL_STRUCT (&spec->data.gtkcustom);
+      break;
+
     case META_COLOR_SPEC_BLEND:
       if (spec->data.blend.foreground)
         meta_color_spec_free (spec->data.blend.foreground);
@@ -1153,7 +1165,67 @@ meta_color_spec_new_from_string (const char *str,
 
   spec = NULL;
 
-  if (str[0] == 'g' && str[1] == 't' && str[2] == 'k' && str[3] == ':')
+  if (str[0] == 'g' && str[1] == 't' && str[2] == 'k' && str[3] == ':' &&
+      str[4] == 'c' && str[5] == 'u' && str[6] == 's' && str[7] == 't' &&
+      str[8] == 'o' && str[9] == 'm')
+    {
+      const char *color_name_start, *fallback_str_start, *end;
+      char *color_name, *fallback_str;
+      MetaColorSpec *fallback = NULL;
+
+      if (str[10] != '(')
+        {
+          g_set_error (err, META_THEME_ERROR,
+                       META_THEME_ERROR_FAILED,
+                       _("GTK custom color specification must have color name and fallback in parentheses, e.g. gtk:custom(foo,bar); could not parse \"%s\""),
+                       str);
+          return NULL;
+        }
+
+      color_name_start = str + 11;
+
+      fallback_str_start = color_name_start;
+      while (*fallback_str_start && *fallback_str_start != ',')
+        {
+          if (!(g_ascii_isalnum (*fallback_str_start)
+                || *fallback_str_start == '-'
+                || *fallback_str_start == '_'))
+            {
+              g_set_error (err, META_THEME_ERROR,
+                           META_THEME_ERROR_FAILED,
+                           _("Invalid character '%c' in color_name parameter of gtk:custom, only A-Za-z0-9-_ are valid"),
+                           *fallback_str_start);
+              return NULL;
+            }
+          fallback_str_start++;
+        }
+      fallback_str_start++;
+
+      end = strrchr (str, ')');
+
+      if (color_name_start == NULL || fallback_str_start == NULL || end == NULL)
+        {
+          g_set_error (err, META_THEME_ERROR,
+                       META_THEME_ERROR_FAILED,
+                       _("Gtk:custom format is \"gtk:custom(color_name,fallback)\", \"%s\" does not fit the format"),
+                       str);
+          return NULL;
+        }
+
+      fallback_str = g_strndup (fallback_str_start, end - fallback_str_start);
+      fallback = meta_color_spec_new_from_string (fallback_str, err);
+      g_free (fallback_str);
+
+      if (fallback == NULL)
+        return NULL;
+
+      color_name = g_strndup (color_name_start, fallback_str_start - color_name_start - 1);
+
+      spec = meta_color_spec_new (META_COLOR_SPEC_GTK_CUSTOM);
+      spec->data.gtkcustom.color_name = color_name;
+      spec->data.gtkcustom.fallback = fallback;
+    }
+  else if (str[0] == 'g' && str[1] == 't' && str[2] == 'k' && str[3] == ':')
     {
       /* GTK color */
       const char *bracket;
@@ -1497,6 +1569,16 @@ meta_set_color_from_style (GdkRGBA               *color,
     }
 }
 
+static void
+meta_set_custom_color_from_style (GdkRGBA         *color,
+                                  GtkStyleContext *context,
+                                  char            *color_name,
+                                  MetaColorSpec   *fallback)
+{
+  if (!gtk_style_context_lookup_color (context, color_name, color))
+    meta_color_spec_render (fallback, context, color);
+}
+
 void
 meta_color_spec_render (MetaColorSpec *spec,
                         GtkStyleContext *style,
@@ -1519,6 +1601,13 @@ meta_color_spec_render (MetaColorSpec *spec,
                                  spec->data.gtk.component);
       break;
 
+    case META_COLOR_SPEC_GTK_CUSTOM:
+      meta_set_custom_color_from_style (color,
+                                        style,
+                                        spec->data.gtkcustom.color_name,
+                                        spec->data.gtkcustom.fallback);
+      break;
+
     case META_COLOR_SPEC_BLEND:
       {
         GdkRGBA bg, fg;
diff --git a/src/ui/theme.h b/src/ui/theme.h
index e248b3b3..56144edc 100644
--- a/src/ui/theme.h
+++ b/src/ui/theme.h
@@ -232,6 +232,7 @@ typedef enum
 {
   META_COLOR_SPEC_BASIC,
   META_COLOR_SPEC_GTK,
+  META_COLOR_SPEC_GTK_CUSTOM,
   META_COLOR_SPEC_BLEND,
   META_COLOR_SPEC_SHADE
 } MetaColorSpecType;
@@ -262,6 +263,10 @@ struct _MetaColorSpec
       GtkStateFlags state;
     } gtk;
     struct {
+      char *color_name;
+      MetaColorSpec *fallback;
+    } gtkcustom;
+    struct {
       MetaColorSpec *foreground;
       MetaColorSpec *background;
       double alpha;