Add some dark mode notes.

Technotes/DarkMode.md

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+# Dark Mode
+
+https://developer.apple.com/documentation/appkit/supporting_dark_mode_in_your_interface
+https://developer.apple.com/videos/play/wwdc2018/210/
+
+Accent colors
+Can do preference for content —  see Mail for example
+linkColor — see if can use in web view
+icons in sidebar should not be vibrant
+Use opaque grayscale colors, not opacity, on top of vibrancy
+Colors in asset catalogs
+	Specify for different  appearances
+	High contrast colors
+Dynamic system colors
+Resolved at draw time
+Pictures in asset catalogs
+Template images
+	contentTintColor new API - NSImageView, NSButton
+Render as template image thing in IB
+controlAccentColor
+color.withSystemEffect(.pressed)
+Avoid nonsemantic materials
+Semantic materials: popover, menu, sidebar, selection, titlebar, etc.
+visualEffectView.material = .popover
+Desktop tinted background: window background, underpage, content background
+contentBackground default for collection views
+Use NSAppearance to override inheritance
+.aqua
+.darkAqua
+effectiveAppearance
+
+Advanced Dark Mode:
+  https://developer.apple.com/videos/play/wwdc2018/218/
+
+Build with 10.14 SDK
+NSAppearanceCustomization
+NSView, NSWindow conforms
+NSWindow.appearanceSource
+