How to make a white-on-black word using Libre Office Writer where the black background includes a substantial margin?

I’m using LO 6.3.2.2 and specifically LO Writer.

In a document which is otherwise black on white, I want to highlight the occasional word by having it in white text on a black background. When I do this simply by changing the highlight colour and the font colour, the black box created is only slightly larger than the white text. Is there a setting I can use to adjust the size of the black box, for example to give it an extra 1mm margin on each side?