You’re not alone, @Chris_Green. The forums are full of other users upset by this change and AppSheet is aware.
They keep mentioning that this is an incremental change and hinting towards some larger improvement, but have not said if this specific option will return.
I like the new Light/Dark options, and I understand the need to roll out major UI updates in phases. I’m really hoping that this is just the first step at giving us much more control over the UI- including changing the header colors again.
AppSheet has been a lot better about listening to community feedback lately. And of all the recent changes, this one seems to have the most negative responses in the forums.
Overall, I’m happy with the platform and think the dev’s are doing great work.
All we can do is keep providing feedback and hope the next update addresses the issues we are most vocal about.