It can be fixed by rotating to landscape mode and then rotating back to vertical mode. The button will be matched with theme you selected (either light or dark mode)
Thank you for your feedback. If you haven’t already, Please update Samsung Members app to latest version from the Galaxy store and send us an error report immediately after experiencing the issue from the Samsung member’s app -> Beta Feedback -> Error Reports to help us debug the issue. We apologize for the inconvenience and appreciate your contribution to the beta program.
One UI Beta Team