Settings → Appearance switches the whole app between light and dark, the same action as the ⌘K palette's theme command. There is no third mode and no per-page override: one choice applies everywhere, immediately.
Theme is a preference kept on this computer, not a fact about your
portfolio. It lives in localStorage, not the settings table, so it is
never written to the database, never travels with a backup or a moved file,
and stays changeable even after a commercial trial ends, the same as the
other display and accessibility preferences.