How can a user apply a theme in PowerPoint?

To apply a theme in PowerPoint, selecting a theme from the "Design" tab is the correct approach. The "Design" tab provides a dedicated area where a variety of pre-designed themes are displayed, allowing users to choose a cohesive look for their presentation. This tab not only offers a range of visual styles but also allows for customization, enabling users to modify colors, fonts, and effects according to their needs.

The other methods mentioned do not provide the same functionality. Right-clicking on a slide typically brings up context-specific options related to that slide but does not provide access to theme selections. Using the "Insert" tab is focused on adding elements like images, tables, or charts and does not pertain to the overall design theme of the presentation. Choosing a color from the palette is more about altering specific elements within the presentation rather than applying a comprehensive themed design to all slides. Therefore, the most efficient and straightforward way to apply a theme is indeed through the "Design" tab.