You must write it this way

.baseStyle {
 /* base styles goes here*/

}

.baseStyle:hover {
/* hover styles goes here */
}

If you wish to attain what you intend you must use scss.

Then it will be like

.class {
    margin:20px;
    &:hover {
        color:yellow;
    }
}

Reference

1. Using Scss in React

If you want to write in same class. You should use Sass/Scss.

Save the css file with .scss extension. It is a precprocessor for better syntax.

Refer this sandbox:

https://stackblitz.com/edit/react-uwktzj?file=src%2FApp.js