What option do users have to change the overall look of their PowerPoint slides?

Applying a theme from the "Design" tab is the most effective way for users to change the overall look of their PowerPoint slides. A theme encompasses a set of design elements, including colors, fonts, and effects that create a cohesive look throughout the entire presentation. By selecting a theme, users can instantly apply a uniform aesthetic to all slides, ensuring consistency and professionalism, which is crucial for effective presentations.

While editing individual slides can change the appearance of those specific slides, it does not provide a holistic change to the entire presentation. Adjusting font size affects text on a slide but does not alter the overall visual style or cohesion of the presentation. Similarly, using transition effects can enhance how slides appear when moving from one to another, but it does not change the fundamental design aspects of the slides themselves. Therefore, applying a theme is the most comprehensive and efficient choice for changing the overall look of PowerPoint presentations.