In the AndroidManifest.xml, under the application tag, you can set the theme of your choice. To customize the theme, press Ctrl + Click on android:theme = "@style/AppTheme" in the Android manifest file. It will open styles.xml file where you can change the parent attribute of the style tag.
At parent= in styles.xml you can browse all available styles by using auto-complete inside the "". E.g. try parent="Theme." with your cursor right after the . and then pressing Ctrl + Space.
You can also preview themes in the preview window in Android Studio.
