Agreed – Logic Pro X still doesn't fully support the native API, for example. But they really seemed to make a show of how all their built-in apps were heralding its arrival; Notes, System Preferences, iTunes, Photos, Safari… even Disk Utility and Activity Monitor. So TextEdit with a white editing area seemed to be where it had landed. (In fact, maybe its chrome was dark originally, and the change was to make the editing area dark by default as well.)