Welcome to GeoNet :)
Hmmm, I don't think a square box around labels is possible. But when you label features you can turn on the Halo, see screen dump below. This Halo will render your labels much more readable.
Thank you. I was looking at that too, but in our case preferred option would be black square background, with yellow text. Discussing other options too, like Story Map. If to build it well, results can be really good.