What should you do to change the default "Menu" block in your OutSystems application?

The correct answer is found in the understanding of how styles are applied in OutSystems, especially in relation to reusable components like blocks. When you want to change the default appearance of a "Menu" block that is used across various screens, you should specifically target the block's own Style Sheet. This approach ensures that modifications are encapsulated within the block itself, promoting reusability and maintainability.

By inserting CSS in the block's Style Sheet, any changes will specifically affect that block wherever it is used in the application. This method isolates the style changes to the block without unintentionally altering other components or screens that may share styles from a broader or different context.

Using the module Style Sheet or the OutSystems UI Style Sheet can lead to unwanted side effects, such as cascading changes that might affect other elements of the application. Additionally, going to every screen to update it would be inefficient and would negate the benefits of having reusable blocks in the first place. Thus, customizing the block directly via its Style Sheet is the most effective and best practice approach within the OutSystems platform.