Greetings extra,
The keyboard shortcut to use wide or narrower margins is tw. Press those letters consecutively when not in edit mode (you can press ‘escape’ to exit edit mode) and the text toggles between being wide and narrow.
The narrower text space reputedly allows easier reading than having text across the page. I use the narrower view even when using Logseq in full screen, but will switch to wide on occasions.
Regards, Gary