Google has finally launched a dark theme to Search on Desktop. Although the change was first noticed in December, it is now officially available and will be rolling out to all users within the next few weeks according to a Google product support manager.
The new, not-quite black theme can be obtained by going to Settings > Search Settings > Appearance. You will then be able to select dark and have the theme automatically updated based on your device settings.
Although the changes are being implemented now, it may take several more days or even weeks to be available to all. 9to5Google also notes that users have noticed a sun icon that allows them to toggle the feature on and off without having to go into the settings page. However, it is not clear if this is an official update or a test.