Make coloring in codeblock-pict more consistent with code

[takikawa]

This PR should make the coloring for codeblock-pict look more like code. Here are some example snippets that will show the current differences if you run them in DrRacket:

(require pict/code)
(codeblock-pict "#:foo")
(code #:foo)
(codeblock-pict "define")
(code define)

[stamourv]

Thanks!