Added parametric guide lines colors

pygal/css/graph.css

@@ -68,10 +68,12 @@
 
 {{ id }}.axis .guide.line {
   stroke-dasharray: {{ style.guide_stroke_dasharray }};
+  stroke: {{ style.guide_stroke_color }};
 }
 
 {{ id }}.axis .major.guide.line {
   stroke-dasharray: {{ style.major_guide_stroke_dasharray }};
+  stroke: {{ style.major_guide_stroke_color }};
 }
 
 {{ id }}.horizontal .axis.y .guide.line,

pygal/style.py

@@ -62,6 +62,8 @@ class Style(object):
     # Guide line dash array style
     guide_stroke_dasharray = '4,4'
     major_guide_stroke_dasharray = '6,6'
+    guide_stroke_color = 'black'
+    major_guide_stroke_color = 'black'
 
     opacity = '.7'
     opacity_hover = '.8'