Review Board 7.1 will now ship with system theme-matching on by default,

instead of light mode. This will enable dark mode if the system is set

to dark mode, and light mode otherwise.
With this, dark mode is no longer in beta.
Unless a user has explicitly changed their theme, they'll use

system theme-matching as the new default. If they've explicitly opted

into either light or dark mode, they'll continue using that instead.

Verified that my default theme was matching my system.
Checked the Appearance tab and saw the options we want to let users

pick from: Default (system theme), Dark Mode, and Light Mode.
All unit tests pass.