1//React Native Button element doesn't have style props and offers very 
2//few customization. Use TochableXXX elements instead.
3//TouchableOpacity works fine
4//Or you can use Button from React-native-elements library
5
6import {
7  TouchableOpacity,
8  Text,
9} from "react-native";
10
11<TouchableOpacity
12          style={styles.button}
13          onPress={this.onSubmit}
14          disabled={!this.state.isFormValid}
15        >
16          <Text style={styles.btnText}>Add
17			</Text>
18</TouchableOpacity>
19
20const styles = StyleSheet.create({
21
22  button: {
23    width: 200,
24    marginTop: 20,
25    backgroundColor: "green",
26    padding: 15,
27    borderRadius: 50,
28  },
29  btnText: {
30    color: "white",
31    fontSize: 20,
32    justifyContent: "center",
33    textAlign: "center",
34  },
35});