More whites drain more juice in a device with amoled screen, Its true
I don't know why but still many designers design UI to be so, Flyme is not the only one doing this MIUI, EMUI ... does the same they have more whites in the UI and no one cares
I'm currently in flyme 6 closed beta and i can tell you the white's got much brighter in flyme 6; But everything isn't in white there are apps that got much darker UI too

Only thing that's too disappointing is flyme 6 is for now based on flyme 5.1