adding style sheet to react component from 7-in-1 pattern code example

Example 1: class in react

class App extends Component {
    constructor() {
        super()
        this.state = {
            name: "Sally",
            age: 13
        }
    }
    
    render() {
        return (
            <div>
                <h1>{this.state.name}</h1>
                <h3>{this.state.age} years old</h3>
            </div>
        )    
    }
}

Example 2: how to add class to element on mouseclick with js

// element.classList.add("newClass");
// or
// element.className += "newClass";

// Example

document.querySelector("button").onclick = function addNewClass() {
  document.querySelector(".elementToChange").classList.add("newClass");
}