Feature Request: Adjust width of text layout to reduce whitespace / padding

[ahoythedinghy]

I love using Bibisco, but a year into it I still find myself looking again every so often to be sure there's no option to reduce the padding to the left and right of the text entry and display area. This is especially true for distraction free editing mode.

The attached images will probably explain my request better than what I'm writing here. The part I've marked in red is what I'd please request an option to adjust, presumably by widening the center area that I've marked in blue.

Thank you!