In Oracle APEX, you can choose from different authentication methods to establish a user’s identity and control access to your application. Authentication may require a user to provide some type of credentials, such as a user name and password, or could involve the use of digital certificates or a secure key. Three of the authentication methods that you can choose in Oracle APEX are:

• Custom Authentication Scheme: This method allows you to create your own authentication logic using PL/SQL code or an external web service. You can also customize the login page and session management for your application.
• No Authentication (using DAD): This method adopts the current database user as the authenticated user. This approach can be used in combination with a mod_plsql Database Access Descriptor (DAD) configuration that uses basic authentication to set the database session user.
• Built-in Authentication Scheme: This method allows you to use one of the preconfigured authentication schemes that ship with Oracle APEX, such as Open Door Credentials, LDAP Directory, Social Sign-In, and so on.

A theme is a collection of templates and stylesheets that define the layout and appearance of an application. You can create a custom theme for an application by using the Create Theme Wizard or by copying an existing theme. If you want to use a custom theme for anotherapplication in the same workspace, you can copy the theme to that application by using the Copy Theme option in Shared Components. However, if the master theme (the original theme) may still require changes, you need to subscribe the copied theme to the master theme and refresh the theme whenever there are changes in the master theme. This solution guarantees that any changes to the master theme are reflected in the copied theme by using a synchronization process that updates templates and stylesheets based on their subscription status. Verified References: [Creating Themes - Oracle Help Center], [Managing Theme Subscriptions - Oracle Help Center]

Shared components are common elements that can be displayed or applied on any page within an application. Once you create a shared component, you can add them to any page within your APEX application. However, shared components are specific to each application and workspace, so you cannot add them to any page in any workspace4. Breadcrumbs, lists, and navigation bar entries are examples of shared components that can be used for navigation purposes4.