A little adding that increase visibility in autocomplete mode.

Hi !

In autocomplete mode background was transparent so I made a little adding that change propover-list's css that, I think, respect the base theme.

Here's what it looks like 
![capture d ecran - 02102014 - 01 24 23](https://cloud.githubusercontent.com/assets/2990689/4484935/6d8e7bd2-49c2-11e4-9be8-68469f69d054.png)