User Menu Actions
User menu actions work with userMenu component through actionItem.
Below are the standard user menu action types.
sec_userMenuSubstituteUser
Action class: UserMenuSubstituteUserAction
Depending on the number of substituted users, either opens a lookup view to select a substituted user or shows a submenu with the substituted users.

You can define the maximum number of users that can be substituted by the currently authenticated user, which shows a submenu instead of opening a lookup view using the jmix.security.ui.substitute-user-action-max-substitutions application property. The default value is 5.
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userMenu_themeSwitch
Action class: UserMenuThemeSwitchAction
Provides a submenu to switch between theme variants: light, dark and system.

userMenu_themeSwitch
requires import of the JavaScript file provides code that applies a theme variant to an HTML document depending on the value in the Web Local Storage and subscribes to the prefers-color-scheme to update the theme variant if the user indicates their preference through an operating system setting (e.g., light or dark mode) or the user agent’s settings.
You need to import it into the main application class:
@JsModule("./src/theme/color-scheme-switching-support.js")
@SpringBootApplication
public class OnboardingApplication implements AppShellConfigurator {