Go to Settings → General. This tab holds preferences that apply across your whole account.
Theme
Switch between light mode, dark mode, or follow your system setting. The change applies immediately.
Default alias domain
When you create a new alias, the domain dropdown pre-selects whatever you set here. If you mostly use a custom domain, set it once here and you won't have to change the dropdown every time.
The selector shows all your active custom domains plus the available brand domains.
Other preferences
General may also include timezone, language, and other account-wide defaults depending on the current version. Changes save automatically or with a Save button depending on the specific setting.