Presentation Slider – Text Background Color

I would like to have a background color behind the text only on the presentation slides on the home page. Basically a colored box behind the display text. Is this possible? The images I’m using vary from lights to darks, so not all of the text is readable without a color behind it.

If it is possible, is it possible to change the opacity of the background box color that is behind the text? So if I had a white or black box, it could be 50% opacity?

Thanks so much!