Theme Tab - Reset Behavior - Introduce an option to reset all user-defined settings and display a theme in its initial configuration

[JaneSjs]

T16845 - When selecting another theme, the customized background and other properties remain between the themes https://surveyjs.answerdesk.io/internal/ticket/details/T16845

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Usage scenario: When a user changes a theme, handle this and reset all user-defined settings and display a theme in its initial configuration.

So, we need the following APIs:

• An event which is raised when a user changes a theme;
• A function/method to programmatically reset a theme. The theme should appear in its initial configuration, e.g., without user-defined settings.