Support Chrome 76 dark mode

[drewbrew]

https://blog.chromium.org/2019/06/chrome-76-beta-dark-mode-payments-new.html

Chrome 76 beta allows the CSS to query whether the user has OS-level dark mode enabled. If so, we should query that and switch to a darker color scheme (I know this requires some substantial theming, so this does not need to be high on the priority list)