Content area styling updates

Fixes #153 

/cc @diegoreymendez 
Hey @bummytime.  I was asked in [this ticket](https://github.com/wordpress-mobile/WordPress-iOS-Editor/pull/213) not to use custom colors, but only colors from `WPStyleGuide`.

Since this issue is from August, and the comment I received was from September, I believe the color request in this ticket may be outdated.

Ping @hugobaeta so he can chime in and propose a good option.