I find the white backgrounds in Material obnoxious. It's as if Google's designers work in well-lit offices and don't test this stuff in dark environments where a white background hurts to look at. Light backgrounds also waste power with AMOLED screens.

I can understand why Google wasn't inclined to include a full theme engine on Android and ask to app developers to support that, however it seems to me that having a system-wide light-mode and dark-mode would be a great feature for devices centered around mobility and therefore not always used in well-lit offices.