After install the 'rc-easyui', the package will be installed in 'node_modules/rc-easyui' directory. You can put this lines to the 'index.css' file that is in your project.
@import '~rc-easyui/dist/themes/material-teal/easyui.css';
@import '~rc-easyui/dist/themes/icon.css';
@import '~rc-easyui/dist/themes/react.css';
And then import the 'index.css' file in your 'index.js'.