Dear Cori,
Good day!!
Apologies for the inconvenience caused at your end.
I understand your concern and when I tested the behavior at my end, I noticed the change is applied to the latest version of OneNote app on Current Channel, please check the picture 1:
However, when I check on the latest version of OneNote app on semi-annual channel, the UI is same as previous, please check the picture 2:
I agree with you on the recent change on the OneNote app UI with the Dark Gray background is not convenient and since the changes are from the development team, I suggest you report the behavior to the related OneNote development team via adding the feedback. Open the OneNote app> Help> Feedback.
I have submitted my feedback on this concern in the OneNote app and we will also try our best to find the related channel to report this behavior.
Apologies for not delivering the required output because of the change made development team and the moderators in this community are with limited resources and hope you can understand.
Appreciate your patience and have a good day!!
Best Regards,
Sophia