work on ace tooltips

Fix an issue where tooltips for warnings displayed yellow text on a white background.

![screen shot 2014-05-08 at 3 11 20 am](https://cloud.githubusercontent.com/assets/1269969/2914968/e4cf0324-d6a4-11e3-9277-a2c65a067642.png)

This CL:

![screen shot 2014-05-08 at 4 25 40 am](https://cloud.githubusercontent.com/assets/1269969/2914972/f1e78284-d6a4-11e3-9938-ad8d0b00f3da.png)

Also fixes #1790.

@umop, @dinhviethoa