React to Mojave dark theme

Created by: felixfbecker

Feature request description

CSS has a standard now for detecting dark mode and it has been implemented in Safari Technology Preview 68.

https://drafts.csswg.org/mediaqueries-5/#descdef-media-prefers-color-scheme

We can read it from JS with https://developer.mozilla.org/en-US/docs/Web/API/Window/matchMedia

We should support three states essentially, "light", "dark" and "OS"/"auto".