First Steps - Introducing Themes

How do I use themes in my app?
This post extends my previous post introducing styles, so if you've not used styles in android before I would recommend reading that first.

A theme is specified for an entire application or individual activities within the manifest. It encapsulates a number of visual style settings, such as text and button styling.

Android has a default theme which is the 'dark' theme, black background with white text, that you will see it you were to open the Settings menu for example. When you create an android app you do not need to specify the theme as the dark theme is used by default.

It is likely that you will at some point want to hide the application title bar, this can be done by setting the theme to Theme.NoTitleBar. Android also has a 'light' theme built in (black on white) which can be used by setting the theme to Theme.Light. You can also chain the theme settings to allow combinations, for example Theme.Light.NoTitleBar. Themes are applied in the manifest as shown below:

Manifest sample:

<application
  android:icon="@drawable/ic_launcher"
  android:label="@string/app_name"
  android:theme="@android:style/Theme.Light">
  <activity
    android:name=".Main"
    android:theme="@android:style/Theme.Light.NoTitleBar">

System styles are prefixed for the @android tag. Much like styles, custom themes are defined within an XML file within your resources and each theme must have a name and can optionally have a parent, for inheriting properties:

res/values/themes.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="Theme.myTheme" parent="android:Theme.Light.NoTitleBar">
     <item name="textSize">20sp</item>
    </style>
</resources>

You can then apply your own theme within your project manifest, as shown below.

Manfiest sample:

...
<application
    android:icon="@drawable/icon"
    android:label="@string/app_name"
    android:theme="@style/Theme.myTheme">
    ...