Allow overflow when centered element goes off screen

On my website https://xansnh.neocities.org/ (similarly with https://xan.nekoweb.org/) everything is centered with a flexbox, but the contents change size to always fit the screen, which doesn't really work for smaller viewports. Or, they might go slightly off screen (above/below), even with a scrollbar.
But subpages on my main website, for example https://xan.lol/#now, all the contents are surrounded by

display: block

display: block

elements with margin.
How can I achieve that kind of margin, but still keep contents centered without scrollbars when they wouldn't normally go off screen or exceed some percentage of the view-height?

Xan on Neocities