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.
Hi @Laura M
I tested this scenario in my own environment using Brand Images in PowerPoint and observed the same behavior. Based on my testing and research, I couldn't find a setting to customize the background color of SVG thumbnails in the Brand Images picker. The available Microsoft documentation doesn't mention this option either.
As a result, there doesn't appear to be a supported way to change the thumbnail background to make white SVG assets more visible.
If this makes it difficult for users to identify white SVG assets, I recommend submitting feedback through https://feedbackportal.microsoft.com/feedback so the product team can review your feedback.
I know this may not be the answer you were hoping for, but I hope this information helps clarify the situation.