React Native buttons are very limited in the option they provide.You can use TouchableHighlight or TouchableOpacity by styling these element and wrapping your buttons with it like this
<TouchableHighlight
style ={{
height: 40,
width:160,
borderRadius:10,
backgroundColor : "yellow",
marginLeft :50,
marginRight:50,
marginTop :20
}}>
<Button onPress={this._onPressButton}
title="SAVE"
accessibilityLabel="Learn more about this button"
/>
</TouchableHighlight>
You can also use react library for customised button .One nice library is react-native-button (https://www.npmjs.com/package/react-native-button)