Slider image text background

One my my main issues with the default slider module is that you have to use images that are dark in order for the white text to show. What would be the best way to either change the text color, or turn the text box background to translucent grey so the text pops out a bit more on white photos?

In other words, what is the css selector for the text box i can use?

selector here { background: rgba(0, 0, 0, 0.5);}