Thanks for the reply! Here are the answers to your questions:
a. body (I think - see the code below please)
b. Yes, the image has already been resized to fit the screen.
c. I want the text box to cover the whole passage, but it should not extend beyond the text, i.e. it should be as small a text box as possible that just manages to encompass the text, and not go beyond.
Using the rgba function you mentioned I modified the values to produce this:
background-position: center center;
background-color: rgba(255,255,255, 0.3);
This is very close to what I want, but right now my third point isn't met - the text box covers space that doesn't have text. Is it possible to limit the text box's size somehow?
EDIT: I figured it out (using width and height). Thanks so much!