This PR adds basic support for macOS 10.14/Mojave's "dark mode", by explicitly disabling the NSRequiresAquaSystemAppearance app bundle setting (via "Info.plist") and fixing the most egregious color/stylesheet-related issues that prevented dark mode from being considered "supported" earlier.

There are additional optimizations that could be made here still (for example, by providing both "light" and "dark" variants of various image files/resources and loading the appropriate variant to further improve contrast as needed) but, for now, Mojave's dark mode should at least be usable.

Closes #213