A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
You can refer to AI's recommendations first, as it already explains the supported file types and how to apply them in PowerPoint.
One additional point that if you want a more dramatic visual change than what you're seeing with the available themes, consider downloading a PowerPoint template (.potx) instead of only a .thmx theme file. Templates can include custom slide masters, layouts, backgrounds, and design elements, whereas themes primarily affect colors, fonts, and effects. As a result, a template often provides a more noticeable redesign of the presentation.
If applying a new theme does not sufficiently change the appearance of the deck, you can start with a new presentation based on the downloaded template and then import your existing slides using Use Destination Theme. This allows the slides to better adopt the new template's design.
Also, for security reasons, it's generally recommended to use .potx files from reputable sources and only use .potm files if you trust the publisher, since macro-enabled templates may contain VBA macros.
I hope this information helps and if you have any further questions, please feel free to ask via comment section.