A ton of Google apps have gotten reskinned with the company's updated Material look, and Contacts was among the first to receive the redesign. Given the apps' very generous use of the color white, many have been begging for a dark theme. Android Messages (or, rather, simply 'Messages' now) and Google News have already gotten dark modes, and Contacts is the latest Google app to be updated with one.
Contacts initially received the new Material interface back in August, and it was further updated last month in v3.1. Version 3.2 finally brings the dark mode that all of you have been whining for. The dark mode toggle is accessible via the pull-out menu. If you're seeing "Dark theme in device Settings is on" where the toggle should be, then you need to disable Night mode in Developer options. There don't appear to be any other changes.
If you'd like to check dark mode out, you can download the Contacts v3.2 APK from APK Mirror right now. You'll probably have to force-close Contacts after updating to see dark mode.