You can create an Announcement from Admin > Instance Settings > Announcements. Announcements appear as a banner at the top of every page until a user dismisses them. Announcements can be used for anything, but generally would be used to alert users to expected down time or changes to configuration.
Announcements can be related to a particular date (e.g. a scheduled upgrade or down time), and you can also schedule the announcement itself to begin and/or end on a given date.
When you first set up your instance you will be directed to the Site Settings page. Here you can edit your basic instance information including name, description, a code of conduct, and other details. This is also where you can upload a logo, set up footer details, and select a default theme.
The Registration page of the Instance Settings section is where you can enable or disable registration, decide whether to require users to confirm their email address, allow or disallow invite requests, and set a question for invite requests (useful for slowing down or identifying automated requests).
BookWyrm comes with default themes, however you can add your own custom theme. Themes should be .scss files.