Investigate Dark Mode support

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Is your feature request related to a problem? Please describe.
One of the features in Android and iOS is what's called "dark mode." In it, the background is set to a dark color, with the text in a lighter color. An example of this is found here: https://webkit.org/blog/8840/dark-mode-support-in-webkit/

Describe the solution you'd like
It would be nice to investigate what's needed to support this in AIM, such that the color scheme displayed in AIM matches the settings on the phone. An alternative implementation would follow what we do for localization, where we allow the user to either follow the device's setting or override it for AIM.

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More info / guidelines: https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/dark-mode/

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According to CanIUse.com, the color-scheme CSS attributes are not supported on Chrome for Android: https://caniuse.com/#search=color-scheme.

At best, we'd need to manually swap out CSS stylesheets -- which is not impossible. The CSS templates we use do have light / dark permutations (http://topcoat.io/topcoat/). But this looks like it would take some work.