The markup toolbar appears to adopt either a light or dark theme - as dictated by the image being edited. The dark theme toolbar does appear to be consistently difficult to see - in particular when overlaid against a black background. By example:

Unfortunately, the light/dark toolbar theme cannot be manually selected - and neither can the toolbar contrast be adjusted to improve legibility of the tools and dynamic tool labels.
You would be best advised to submit appropriate comments as a Bug Report via Apple's Product Feedback portal:
Feedback - iPad - Apple
You might also contact the Apple Support Team to outline the problem using the Support link at top-right or bottom-left of this page. Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044