That's an interesting use case! When I first read the title, I thought the new and improved image UI coming in v1.10 should definitely have you covered.
But it looks like this use case is actually a little trickier and would require an additional setting to show/hide labels. This is probably useful anyways as a toggle in the toolbar, so you can get a better overview of all of your boxes.
In the coming v1.10, you could then also adjust the stroke width of the boxes (maybe 1 or 2 pixels so it covers less), hide the labels and annotation would be much easier. I also want to add support for custom label-specific colors, which would allow you to allocate one specific color to the label DIGIT, so it's always clear what's what, even without labels.
In the meantime, if you need a quick workaround for the current interface, you could do something like this in your "global_css":
.prodigy-content svg text { display: none }
.prodigy-content svg path { stroke-width: 2px }
This will hide all labels and use a thinner border for the bounding boxes. (The only thing that's unideal is that it won't let you easily tell apart DIGIT from READING anymore.)
Also just found this thread, which has an alternative approach for toggling boxes: Add possibility to toggle all boxes in image.manual