Not sure why Blocs uses .modal-fill @norm ? When there is a BS class for full screen modals. .modal-fullscreen. Unfortunately, we can’t select the right part of the markup in the layer tree to add it.
With .modal-fill, you can adjust the max-width to 100% to remove the gap horizontally. The vertical gaps can be removed by adjusting .modal-content.
You might have to set a margin 0 on .modal-dialogue also ? It’s been a while. So just from memory.