Set text color to a black, sans serif font

Hey, here’s another pull request/suggestion to set the text in the tooltip to a black, sans-serif font.

Currently the tooltip text use the color of `span` or `a` defined by the application i18n_viz is running on. In this particular case the text was white on a white background.

Before:

![before](https://f.cloud.github.com/assets/30716/107024/c085f06a-6a1a-11e2-9f06-9c043fdbc8a2.png)

After:

![after](https://f.cloud.github.com/assets/30716/107013/60edb142-6a1a-11e2-8be1-8083ab425c8b.png)

Thanks!