A theme is a style that’s applied to an activity or application so that your app has aconsistent look and feel. It controls such things as the color of the activity background and app bar, and the style of the text.
Android comes with a number of built-in themes that you can use in your apps. Some of these, such as the Holo themes, were introduced in early releases of Android, and others, such as the Material themes, were introduced much later to give apps a more modern appearance.
But there’s a problem. You want your apps to look as modern and up-to-date as possible, but you can only use themes from the version of Android they were released in. As an example, you can’t use the vative Material themes on devices that are running a version of Android older than Lollipop, as the Material themes were introduced with API level 21.
The Android team solved this problem by coming up with the idea of Support Libraries.
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