Increase font contrast, use code highlighting theme of golang.org.

Resolves #297.

Use green for comments. It's consistent with golang.org and has higher
contrast (against background).

Use slightly darker black for code. It's consistent with golang.org and
has higher contrast.
diff --git a/gddo-server/assets/site.css b/gddo-server/assets/site.css
index 0e61939..a2637f8 100644
--- a/gddo-server/assets/site.css
+++ b/gddo-server/assets/site.css
@@ -31,11 +31,12 @@
 code {
     background-color: inherit;
     border: none;
-    color: inherit;
+    color: #222;
     padding: 0;
 }
 
 pre {
+    color: #222;
     overflow: auto;
     white-space: pre;
     word-break: normal;
@@ -49,7 +50,7 @@
 }
 
 pre .com {
-  color: rgb(147, 161, 161);
+    color: #006600;
 }
 
 .decl {