import React from 'react';
import './MyComp.css'

export default class MyComp extends React.Component {
  render() {
    return (
      <div className="wrapp">
        <div className="greenBlock">
          <h1>green </h1>
        </div>
        <div className="yellowBlock">
          <h1>yellow </h1>
        </div>
      </div>
    );
  }
}

.wrapp,
html,
body {
  height: 100%;
  margin: 0;
}

.wrapp {
  display: flex;
  flex-direction: column;
}

.greenBlock {
  background-color: green;
  flex: 0.25
}

.yellowBlock {
  background-color: yellow;
  flex: 0.75
}
```