You are correct that this feature previously existed in Planner - as we consolidated the To Do, Planner and Project experiences into the new Planner we had to make decisions on which features to ship in the first version, so custom backgrounds was not part of that initial release. Bringing this feature back alongside others like the Board View in My Day/My Tasks and Export to Excel was a result of evaluating customer feedback to identify the top functionality to bring back from our previous Planner experience (you can imagine it's tough to bring back all the capabilities we had before across all 3 of these apps - we try our best, but there's lots of competing priorities to sort through!) Thanks for the question Ami_diamond!