remove colors from main stylesheet

They won't work for light and dark styles

static/markdown.css

@@ -1,25 +1,15 @@
 .source.gfm {
-  color: #ccc;
   -webkit-font-smoothing: antialiased;
 }
 
 .gfm .markup.heading {
-  color: #eee;
   font-weight: bold;
 }
 
-.gfm .link {
-  color: #666;
-}
-
 .gfm .bold {
   font-weight: bold;
 }
 
 .gfm .italic {
   font-style: italic;
-}
-
-.gfm .variable.list {
-  color: #666;
-}
+}