customize Android Facebook Login button

You can use styles for modifiy the login button like this

<style name="FacebookLoginButton">
    <item name="android:textSize">@dimen/smallTxtSize</item>
    <item name="android:background">@drawable/facebook_signin_btn</item>
    <item name="android:layout_marginTop">10dp</item>
    <item name="android:layout_marginBottom">10dp</item>
    <item name="android:layout_gravity">center_horizontal</item>
</style>

and in layout

<com.facebook.widget.LoginButton
        xmlns:fb="http://schemas.android.com/apk/res-auto"
        android:id="@+id/loginFacebookButton"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        fb:login_text="@string/loginFacebookButton"
        fb:logout_text=""
        style="@style/FacebookLoginButton"/>

In order to have completely custom facebook login button without using com.facebook.widget.LoginButton.

According to facebook sdk 4.x,

There new concept of login as from facebook

LoginManager and AccessToken - These new classes perform Facebook Login

So, Now you can access Facebook authentication without Facebook login button as

layout.xml

    <Button
            android:id="@+id/btn_fb_login"
            .../>

MainActivity.java

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        FacebookSdk.sdkInitialize(this.getApplicationContext());

        callbackManager = CallbackManager.Factory.create();

        LoginManager.getInstance().registerCallback(callbackManager,
                new FacebookCallback<LoginResult>() {
                    @Override
                    public void onSuccess(LoginResult loginResult) {
                        Log.d("Success", "Login");

                    }

                    @Override
                    public void onCancel() {
                        Toast.makeText(MainActivity.this, "Login Cancel", Toast.LENGTH_LONG).show();
                    }

                    @Override
                    public void onError(FacebookException exception) {
                        Toast.makeText(MainActivity.this, exception.getMessage(), Toast.LENGTH_LONG).show();
                    }
                });

        setContentView(R.layout.activity_main);

        Button btn_fb_login = (Button)findViewById(R.id.btn_fb_login);

        btn_fb_login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                  LoginManager.getInstance().logInWithReadPermissions(MainActivity.this, Arrays.asList("public_profile", "user_friends"));
            }
        });

    }

@Override 
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    callbackManager.onActivityResult(requestCode, resultCode, data); 
}