Outpost ships five themes, one of them light. The instance owner picks the default, and anybody can override it for themselves, because how a screen looks depends on the room you are sitting in as much as on the community you are in.
If you own the instance, the theme is in instance settings alongside the name, description and icon. That choice is branding: it is what every member sees unless they choose otherwise, and it is what a visitor sees on the login screen before they have an account.
User settings, Appearance. Leave it on Instance to follow the owner's choice, or pick any theme for yourself.
Your choice is stored on the machine you are using rather than on your account, which is the same treatment audio device selection gets. The same reasoning applies: this is how you want this screen to look, and your desktop at night and your phone at lunchtime are not the same question.
Also in Appearance. Comfortable is the default spacing. Compact tightens it so more of the conversation fits on screen, which is worth trying on a laptop or when you are following a busy channel.
Most chat apps are dark and stay dark. Daylight exists because a dark interface in a bright room is harder to read, not because light mode is a checkbox to have. If you use Outpost at a desk near a window, try it before deciding you prefer dark.
