suggestion: add background-color to v-navigation tooltip

So the popup may be seen over similar-color elements.  Also is a subtitle pointer.
Before, the tooltip may be hidden by the arrows & text:
![idea-fullpage-tooltip_no_background](https://cloud.githubusercontent.com/assets/1308419/3674753/4889bfe8-1279-11e4-9c82-ee62f2e5c91f.png)
Now it the popup text is easily seen, even with image backgrounds:
![idea-fullpage-tooltip_background](https://cloud.githubusercontent.com/assets/1308419/3674762/61c9d2fe-1279-11e4-8c8f-e0b025a417e8.png)
![idea-fullpage-tooltip_image_background](https://cloud.githubusercontent.com/assets/1308419/3674769/66d213f6-1279-11e4-82ab-1e6b5ac78677.jpg)