It's not a dumb question! I just realized not long ago that it could be done.
Open a text box, plug in your copy and click the arrow so that the box moves to the right side of the page. At that point, you'll be able to check a little box at the top of the textbox, selecting either gray or blue as a background color.
You can't add a background color if the textbox goes completely across the page or is on the left, just as you can't choose a background color for Amazon products and tables that are not on the righthand side of the page.
Hope I explained that clearly. If I didn't, send me a message and I'll try again.