Dark mode for the BitBucket UI

Recently MacOS Mojave improved and expanded the dark mode and it was promoted heavily in media. Many apps followed, introducing dark mode option in their interfaces.

BitBucket interface should also offer a dark mode - the pure white backgrounds look jarring against the latest dark wallpapers and dark toolbars and dark Chrome's menu bars.

Let's add a new feature which would let BitBucket customers switch to a dark UI.

It could be a simple toggle switch somewhere in the sidebar, for example: