For Activate Dark Mode in Contact App, Open Google Contact app and then click on left-top corner have menu button. click on menu button. Now, You can see that Settings option in below image. click on Settings option. So, now active page is settings. in this page, have Theme option. Click on Theme option. In Theme, You can choose 3 option Light, Dark, set by battery. For activate dark theme choose Dark option. Now, Your contact app is being dark.