Setting Site Width with Blocks

I used blocks to design off the 2023 theme and I have it mostly the way I want it but it seems to default to 100% browser width and I want to set it to a fixed width but I cannot find any settings to accomplish this. I can set the widths of individual blocks but not for the site as a whole. The header and footer don’t seem to have width settings nor does anything in between. I’ve managed to get the columns all set to the width I want but they just sit on the far left of the page. I can’t even center them, not that I would want to with the header and footer stretching out beyond them. Anyway, I can’t seem to find anywhere to add in “Additional CSS” to make those changes as everything takes me back into the block editing area.