Use light text color on dark colors in `style.background_gradient`
I'm working on a commit to enable light text color (white by default) on dark barckground when using `style.background_gradient`. This improves readability, without having to limit the color range with the `low` and `high` arguments.

I'm not completely sure yet about the argument name, which is `light_text` at the moment. Any suggestions?

Example:
<img width="793" alt="screen shot 2018-05-31 at 09 49 53" src="https://user-images.githubusercontent.com/44946/40769726-05b49486-64b9-11e8-9bf6-e4ebae2f964c.png">

