Support macOS dark mode system preference (10.14 Mojave and later)

[jpgill86]

Presently (January 2021, with neurotic 1.5.0), if neurotic is installed on macOS manually via conda-forge, the app seems to completely ignore the OS dark mode setting. However, if the same version is installed using an installer made by constructor, the resulting app will respect the OS light/dark mode setting.

In the latter case, if the OS is set to dark mode when the app launches, many elements will display differently. Although I think this will look great when it's all put together properly, presently not all elements adapt to dark mode. For example, the status bar and the epoch encoder toolbar both appear with white text on a light gray background, and the epoch encoder table buttons are black on dark gray. Furthermore, unless neurotic's "dark theme" is toggled, the plot backgrounds remain light gray; when it is toggled, neurotic's choice of dark gray for the plot backgrounds is perhaps too dark to match macOS's dark mode.

Further reading:

• https://doc.qt.io/qt-5.12/macos.html
• https://developer.apple.com/documentation/xcode/supporting_dark_mode_in_your_interface?preferredLanguage=occ