CSS: Use alpha transparency for code background

Code blocks looked terrible in notes.

before:
![solid background](http://cl.ly/image/461O3X1Q1z2c/Screen%20Shot%202014-07-17%20at%208.45.01%20PM.png)

after:
![alpha transparency](http://cl.ly/image/1b0p1Y0v1H08/Screen%20Shot%202014-07-17%20at%208.44.46%20PM.png)

Another option would be to specify `color` for `code`, but I think this works better.

(ignore the typo in the screenshots)
`rbga()` is IE9+. You might want to leave the first `background-color` rule in there as a fallback.