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HiJ. Addo NHS,
As per the description, it seems like the background setting is no longer available after the recent Microsoft Planner update, but only with white background as default from my testing for you just now.
If my understanding is correct, from my search in the community, I found there also have some other users who are having the same requirement as yours to restore the background setting, please check: How do I change backgrounds in Microsoft Planner? - Microsoft Community
In this situation, I understand the inconvenience that now the plain white background can cause worse accessibility and readability issues, and it is necessary to have the background setting back to change color or even a custom picture. Hence, I suggest you raise your own voice directly to the Planner product development team via the Feedback button in Microsoft Planner or Planner · Feedback Community where the product team will check 24/7 and get informed of the real customer requirement to know how to improve a better user experience after the recent Planner update.
Please correct me if I made any misunderstanding or if there have anything else I can do for you.
Your patience is highly appreciated. Hope you a nice day and stay safe!
Best Regards,
Mia