How to finish Android App layout?

In the part of the code, to customize a layout or widget, change the drawable of the backgrounds by another one with images exchanged.

For example, in a EditText you can change the background (both in the xml and in the code) to one with rounded edges (in the state with or without focus) and it will look like app that you passed as a reference. Similarly it was done with the buttons, the toggles of this app.

If, for example, you want to apply changes to all buttons in the app, just create a Theme for your app to change the background of all buttons.

In Layout XML

<EditText 
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" 
    android:background="@drawable/meu_edittext_customizado"/>

No Drawable my_edittext_customizado.xml

<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:state_window_focused="false" android:state_enabled="true" android:drawable="@drawable/meu_edittext_default" />
    <item android:state_window_focused="false" android:state_enabled="false" android:drawable="@drawable/meu_edittext_desativado" />
    <item android:state_enabled="true" android:state_focused="true" android:drawable="@drawable/meu_edittext_ativado" />
    <item android:state_enabled="true" android:state_activated="true" android:drawable="@drawable/meu_edittext_com_foco" />
    <item android:state_enabled="true" android:drawable="@drawable/meu_edittext_default" />
    <item android:state_focused="true" android:drawable="@drawable/meu_edittext_desativado_com_foco" />
    <item android:drawable="@drawable/meu_edittext_desativado" />

</selector>

• Step 1:

For mobile applications the Layout part has its beginning in UX Design : The part where the experience you are expected to have when you use the application.

• Step 2:

After that we apply the UI Design : The part that applies the experience defined in UX Design through visible components such as buttons, tables, animations etc.

Be careful not to confuse: UX x UI

• Step 3:

The programming part is important at the time of construir the application that was designed and defined in the previous steps, because if the programmer can not do it will have been in vain any previous effort.

Edit: On the developer site Android shows the design principles focused on the platform.

As the question without itself does not raise a doubt related to programming, it has no way of responding by giving examples of codes, libraries, etc.