Certain applications offer some kind of way to change the basic color scheme of the UI. However, this is hardly common practice. Instead, most applications simply force you to use a default look despite the fact that we Android users love the freedom inherent to our platform of choice..
If you’ve ever wondered about changing the look of your favorite apps on-the-go without recompiling, we have some good news for you. Thanks to an Xposed Framework module by XDA Senior Member SArnab©®, it’s now possible to change the look of applications like WhatsApp without nasty smali edits. The Ex Themer 2 module fully integrates with previous versions of Ex Themer, and it allows you to modify various UI elements like the status bar, navigation bar, and framework. In the current version, the only supported app is WhatsApp, but this list will undoubtedly grow in time.
To use this module, you obviously need to have root access, as well as the latest version of Xposed Framework. And after applying a theme, you must reboot your device to have the theme display properly.
If you wish to change the visuals of your OS, head over to the module thread and give Ex Themer 2 a shot._________