Unfortunately the answer is no, you can't set a paragraph background color in Scribus (as of version 1.4.5, which is the current stable).
The workaround is to create a rectangle with the desired fill and place it behind the text. When I need to do this
- Create a separate layers for background shapes and for text
- Wait until your text is finalized as much as possible, since it's annoying to keep adjusting background shapes
- Make the baseline grid visible and use that as a guide. I generally make a page grid smaller than my baseline (for example, if my baseline is 15, I might do a grid of 5 or 3 so I have something to snap to around the paragraph so that there is uniform "padding"
Note: According to this closed bug, you can change the background in 1.5.1. I don't have that installed so I can't test.
Edit: I just checked Scribus 1.5.1, and indeed you can set the background for a paragraph style. So it seems that the solution is to upgrade. While 1.5.1 is listed as "developmental", it seems to be stable and reliable enough for production needs if you need the extra features (and there are a lot of improvements to 1.5.x), so upgrade if you can.