There are currently up to 3, sometimes 4 Themes available for Android devices (.Light variations and similar not included)
Theme
the default for the earliest versions of Android up to 2.3 Gingerbread(10), including some minor style changes in those versions
Theme.Holo
introduced with Android 3.0 Honeycomb (11)
Theme.Material
new in Android 5.0 Lollipop (21)
Theme.DeviceDefault
(Could be anything)
introduced with 4.0 Ice Cream Sandwich (14), a theme that can be customized by the device manufacturer. It represents the native look of the device - i.e. Holo or Material on Nexus devices (& in Android Studio's design editor), maybe something custom on other devices. In case "something custom" isn't an option, those devices must come with the stock themes. Apps that want the stock theme have to specify it though.
What is the best approach to this?
No theme + targetSdkVersion >= 14
The simplest, but not necessarily best option is to define no theme at all. Android will then select the default for you. But Android does not want to surprise your app with themes you're not expecting so it falls back to the Theme you probably had designed your app for. It does so by looking at the android:targetSdkVersion within AndroidManifest.xml (which can nowadays be set via gradle).
- Apps that target old platforms, which had only
Theme (i.e. API levels 3-10), will get only Theme.
- Apps targeting 11-13 get
Theme.Holo.
- 14 or above will get
Theme.DeviceDefault.
Since this is just for backwards compatibility, you won't get Theme.Material on your old Gingerbread phone. Therefore no theme + target 14+ = device default.
Specifying different themes
Android's resource overlay system allows to specify styles based on device API level. For example different versions of a style in res/values-v11 and res/values-v21. This is also what any newly created apps via Android Studio will setup for you.
As an example, the most basic setup for a .Light themed app looks like this:
/res/values/styles.xml is applied to every device and serves as base
<resources>
<style name="AppTheme" parent="android:Theme.Light"/>
</resources>
/res/values-v11/styles.xml is loaded on all devices that have API level 11 and above (including those that are 21 and above). But just the newest version of "AppTheme" is actually used.
<resources>
<style name="AppTheme" parent="android:Theme.Holo.Light"/>
</resources>
/res/values-v21/styles.xml
<resources>
<style name="AppTheme" parent="android:Theme.Material.Light"/>
</resources>
Note: alternatively specifying Theme.DeviceDefault in /res/values-v14/styles.xml should be enough for having a default look but that doesn't allow to fine tune the design. Doesn't hurt to add the v14 override. DeviceDefault and Holo could be different after all.
AndroidManifest.xml is the place where the custom theme is put to use. E.g. as application wide default:
...
<application
android:theme="@style/AppTheme"
...
Links to official documentation
Select a theme based on platform version - Official doc for defining different styles via resource configurations
Holo Everywhere - blog post that explains the default mechanism and DeviceDefault / Holo theme
Using the Material Theme - material documentation
