And technically everything works just fine. But when labeling the individual digits it gets annoying and unprecise really fast. I already read about custom UIs and so on, but neither I understood how to do so nor I am sure if this is the best solution for my problem.
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":
Just released Prodigy v1.10, which includes the new and revamped manual image UI that supports moving and resizing bounding boxes, as well as an option to toggle the visible labels (and a keyboard shortcut so you can select boxes without labels).