rdoc: Colorize background of code/pre [ci skip]

Borrowed the style of code/pre from bugs.ruby-lang.org.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67387 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -62,6 +62,9 @@ h6:hover span {
 code,
 pre {
   font-family: "Source Code Pro", Monaco, monospace;
+  background-color: rgba(27,31,35,0.05);
+  padding: 0em 0.2em;
+  border-radius: 0.2em;
 }
 
 /* @group Generic Classes */