Set background image to full screen in Reactjs

Use vh and vw properties:

const styles = {
    container: {
        backgroundImage: `url(${backgroundImage})`,
        backgroundPosition: 'center',
        backgroundSize: 'cover',
        backgroundRepeat: 'no-repeat',
        width: '100vw',
        height: '100vh'
    }
};

Try using the specific property values.

In Index.css:

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

In Login.css:

.Login-component {
    /* The image used */
    background-image: url(../static/login-bg.jpg);

    /* Full height */
    height: 100%; 

    /* Center and scale the image nicely */
    background-position: center;
    background-repeat: no-repeat;
    background-size: cover;
}