Rollup merge of #40265 - wesleywiser:rustdoc_style, r=GuillaumeGomez

Rollup merge of #40265 - wesleywiser:rustdoc_style, r=GuillaumeGomez

Improve the style of the sidebar in rustdoc output

Makes the sidebar a light grey and highlights the currently viewed item in the sidebar more prominently.

All visual design credit goes to @johnwhelchel (#37856)

Sample screenshots:

![screen shot 2017-03-04 at 12 29 48 pm](https://cloud.githubusercontent.com/assets/831192/23580829/db6c26c2-00d6-11e7-8d89-822e25ba79f0.png)

![screen shot 2017-03-04 at 12 30 10 pm](https://cloud.githubusercontent.com/assets/831192/23580828/db69eeca-00d6-11e7-9f89-1e06fd3bf098.png)

![screen shot 2017-03-04 at 12 30 31 pm](https://cloud.githubusercontent.com/assets/831192/23580830/db6d00ce-00d6-11e7-89ca-cd03e148a121.png)