Tap ‘Customize’ in the menu that appears. Select ‘Dark’ to choose dark mode icon colors. (Or, Automatic to have dark mode icons only at nighttime). Why do not all app icons change color?

In the Adjust Color window, drag the slider on Saturation, Temperature, and Tint to change the colors accordingly. Note that you need to do this for every icon size in the stack, from one through 10.

Android 16 Beta 2 prevents setting color-themed icons from “Wallpaper & Style” due to a bug Android 16 Beta 2 just arrived yesterday, but it looks like it already needs to get a hotfix.