Most people are familiar with Material design, but not all Google designed used to be flat!

Kitkat

The last iteration of the older Holo theme

Kitkat Kitkat dark
kitkat kitkat-dark

Material 2

Material 2 has been recently replaced by Material You, but remains in parts of Chrome OS

Chrome OS Chrome OS dark
chrome-os chrome-os-dark

Material You

Characterized by dynamic colors and generous padding, Material You brings Google’s latest design to your desktop

Android Android dark
android android-dark