mobile navigation color and borders

addresses various issues in JIRA https://optimizely.atlassian.net/browse/MKT-795.  I had set a background color in /mobile and /android that was breaking the mobile nav background.  Also, for signed in utility mobile nav some border color changes were missed.  I also made the border colors slightly opaque and created a SASS variable for these colors.

![screen shot 2014-11-18 at 8 28 25 pm](https://cloud.githubusercontent.com/assets/4656726/5100992/607b335e-6f65-11e4-9a92-04230e862698.png)
![screen shot 2014-11-18 at 8 28 18 pm](https://cloud.githubusercontent.com/assets/4656726/5100993/60933760-6f65-11e4-897f-5220c56f7f64.png)