Improved background handling

Author: FlorianRappl

src/AngleSharp.Css/Declarations/BackgroundDeclaration.cs

@@ -124,7 +124,7 @@ public ICssValue Convert(StringSource source)
                         clip));
                 }
 
-                return new CssBackgroundValue(new CssListValue(layers.OfType<ICssValue>().ToArray()), color);
+                return new CssBackgroundValue(new CssListValue(layers.OfType<ICssValue>().ToArray()), color ?? new CssInitialValue(InitialValues.BackgroundColorDecl));
             }
 
             public ICssValue Merge(ICssValue[] values)

src/AngleSharp.Css/Values/Composites/CssBackgroundValue.cs

@@ -43,7 +43,7 @@ public String CssText
                 {
                     var layer = _layers.CssText;
 
-                    if (_color != null)
+                    if (_color != null && _color is CssInitialValue == false)
                     {
                         var sep = layer.Length > 0 ? " " : "";
                         var color = _color.CssText;