For me, like creecros already linked, it was the Customizer, but also the Nebula theme, if I remember correct.
In any way: this issue made me write a new plugin to modify the new dark theme a bit (some contrasts are bad, imo). I will try to keep you people updated in my plugin thread, if I will be able to finish the plugin at some point hopefully. Some teasing ahead: it has a config area where you are able to enable or disable certain aspects of the new styling. 