If custom themes are something you want on Android, there are apps out there that can theme your phone for you, although more extensive themes with system tweaks might require a rooted device. However it seems that Google is finally about to introduce custom themes to Android, making it easier for users to switch around.
As spotted by Mishaal Rahman at XDA, it looks like Google could be getting ready to introduce custom themes to Android O. As you can see in the screenshot to the right, under the Display settings page in the Android O Developer Preview, you have the option of selecting Device Theme which brings up the popup menu you see.
Right now there are only two themes to speak of, but it has been suggested that this could be Google getting ready to introduce the feature to Android O that might make its debut in the final build. However we suggest that users not get their hopes up. This is because in the past we’ve seen Google introduce features only to take it away at the very last minute.