The Settings tab is short on purpose. There are three options.
Default image provider:
Which provider generates page images when no character is locked. Pollinations is the default because it works immediately with no setup. Switch it once you have added premium keys and tested them. Note that story books, and any project with an approved character sheet, automatically prefer a reference capable provider, Google or FLUX, when a key exists, regardless of this setting.
Paper type:
White or cream. This is not cosmetic. It changes the per page thickness used to calculate the spine width on your wraparound cover, 0.002252 inch per page for white and 0.0025 inch per page for cream. Pick the same paper you will select at upload, or the printed spine will not line up with the artwork. White is standard for coloring books, cream is common for story books and journals.
Color scheme:
Ten admin themes, the same set shipped across the whole plugin range, from Results Amber through Clean White, Ivory Cream, Snow Light, Developer Dark, Midnight Blue, Ocean Blue, Sunset Warm, Rose Pink and WordPress Native. This is purely how the admin screens look and it has no effect on generated books.
Where are the print settings?
There are none to set, deliberately. Trim size, bleed, margins and the gutter ladder are derived from the book type and the page count, and they are applied at export time. The only print decision left in your hands is the paper type, because only you know which one you will pick at the printer.
Click Save Settings after changing anything. Paper type takes effect on the next cover build, so if you change it after exporting, run Build Export Files again.