CSS patches: make fileboxed racketblocks & codeblocks look neater

In #lang scribble/manual, when @filebox is wrapped around @racketblock or @codeblock, it looks broken compared to, say, @racketmod. This is due to the HTML rendering being different. This CSS patch smooths over the differences. 

Example: this code

@codeblock|{
#lang pollen

◊define[greeting]{Hi there}.
}|


@filebox["foo.html"]{
@codeblock|{
#lang pollen

◊define[greeting]{Hi there}.
}|}


@racketblock[
(define greeting
  "Hi there")
]

@filebox["foo.html"]{
@racketblock[
(define greeting
  "Hi there")
]}

Under the current CSS, looks like this:

![screen shot 2014-09-01 at sep 01 12 14 27 pm](https://cloud.githubusercontent.com/assets/1425051/4111597/d866aae2-320c-11e4-920a-92c5d64c3f77.gif)

But with this CSS patch, looks like this:

![screen shot 2014-09-01 at sep 01 12 14 40 pm](https://cloud.githubusercontent.com/assets/1425051/4111599/e2d45470-320c-11e4-9285-589ff9b01cf1.gif)