Customization is one the best thing about Android. You can modify your Android UI in every possible way, for example, you can use IOS and Windows like interface, or any other theme which suits you better.
You can change your Icon, fonts, home screen and much more.
Launchers and Custom Widgets are the easiest way to theme your Android Interface.
Google Play store is full themes for Android which are totally free to Download. I have selected these 20+ Best themes for Android.

Also read:

Best Xposed Modules for Android

Best App for Rooted Android Device.

20+ Best Themes for Android.[Free]

Rootless Pixel Launcher with O8 icon pack

Nothing can beat the Stock Android looks. It is very simple, smooth and looks beautiful. Rootless Pixel Launcher is the best because it comes with “At a Glance” widget and Google feeds, which cannot be found in Nova or any other launcher. It also allows you to choose a Icon pack which Google now launcher lacks.

Buzz Launcher

Buzz Launcher is a launcher that enables you to apply shared Homescreens to your own smartphone. Buzz Launcher is an ad-free custom launcher that has more than 1000,000+ free themes. All themes are created by users and professional. With Buzz Launcher, you can Share home screens on Facebook, Twitter, and Google+.

One great thing about buzz launcher is that the themes are highly customized. If you have used Themer but droped it due to no update since 2015 then Buzz launcher is a perfect alternative.

You can find Buzz launcher themes on Homepackbuzz. Here are some themes to start with:

Flat UI – X Edition

Flat UI 5th Edition

Motive Flatta

Simple night premium

Minimal 11.1

Few movie characters themes:

Nova Launcher

Nova Launcher is one the most downloaded and used launcher for Android. With a Near Stock UI, minimalistic design and smooth performance, Nova launcher is a perfect fit for Stock UI lover. Some of the features in free Version are the Custom grid, Color controls (for labels, folders, unread badges, drawer tabs and backgrounds), scrollable dock, scroll effect and custom icons.
You can also make your own theme by using icon and wallpapers. Check out these 10 Best Nova Launcher themes

Zooper Widget

Zooper widget is an app which gives you the power to customize your phone. It’s a DIY widget with limitless customization. Zooper comes with many templates to start with. There are hundreds of widgets for Zooper to choose from. And if you like to make your own then there are some templates which are already included.

Best Widget for Zooper:

MIN for Zooper Pro: This widget contains many Minimal widgets.

Vico: Contains many widgets, mainly of 4X4 area

Reduct-Zooper Template/Skin

Minimal Clock Widget – Zooper: – A Minimal Clock widget with a clean and simple design.

UCCW

Ultimate Custom Widget, is a widget that lets you place custom clocks, battery meters, weather info, and anything else right on your home screen. There is a sheer number of skins available on Google Play and shared in various forums, and more skins are being added all the time.

Find all the Skins for UCCW here.

Go launcher

GO Launcher is an app with its own unique UI, which is much more colorful than the stock one.
Go launcher has its own Theme store from which you can choose any theme. There are 10,000+ themes available to download  along with wallappers, transition effect and widgets. Go launcher also give some extra features like Cleaner or booster and App locker.

ZT.art provides a lot of themes for Go launcher, here are some of the best:

Color Box 2 In 1 Theme

Glorious GO Launcher Theme

Decent GO Launcher Theme

Album GO Locker Theme

Lion GO Launcher Theme

Launcher 8

If you Love the UI like of Windows Phone then this app is for you. Launcher 8, is an app to use Windows phone UI style on your Android devices. It will make your Android phone’s home screen looks like a Windows Phone. You can also customize the layout.

C launcher

C launcher is another launcher which offers great customization, themes, and wallpapers. C launcher also gives you the ability to hide an app and unhide it anytime. This app also contains Widget, like fast cleaner, weather clocker, news, search bar and more.

Best Android Themes Workshop provides themes for C launcher:

Street Night City View Theme

Geometric C Launcher Theme

Triangular Abstract Theme

Geometric Blue Theme

Vibrant Heart C launcher Theme

Themer

Themer is an app with free themes that include HD wallpapers, icon packs, and widgets. The  Home screen personalization is where Themer shines. There are hundreds of free themes and they are unique and Beautiful. You can also Customize them according to your need. It is not updated since October 2015 so there can be some optimisation issues with latest Android versions.