Android give us the opportunity to specify the format and the design of View components or screens of our app, by declaring some characteristics such as color, margin, text size etc. This is happening via a Style, which is a XML resource and it is separate from XML layouts. It isn’t wrong to say that a style is the Android’s css, because it separates the design from the content. Theme is nothing more than a style that applies to the whole app or a certain Activity. In our example we will find out how to create and set our own styles and how to define a theme to our entire application.