Instance-wide default themes (Admin>Setting>Branding) controls the landing page, is set as the default themes for all new accounts and shows up to users as the "Server theme" for light and/or dark theme. If there's no instance-wide themes, there's no "Server theme" shown to the users.
While it controls the landing page theme and user default theme, users can still override it through their account theme management.
Any theme copy-pasted into instance-wide default themes MUST start with the {id:...} property or else it will not take, unlike "Install a theme" from user settings, which is seems more flexible. Some themes copied directly from the theme webpage does not start with the {id:...} so it should be double-checked if used for instance-wide function.
The landing page's background color (left side) is controlled by the bg: (background) property and is easily editable, it balances against the fg: (foreground) setting for readability.
Single-user instances can combine crazy "Instance-wide default themes" with preferred user set themes for a fancy landing page and good user experience/readability. How this effects the onboarding experience for new users is unknown.





