It's described in the official documentation.

In a .d.ts file add this:

import 'styled-components';

declare module 'styled-components' {
  export interface DefaultTheme {
     borderRadius: string;
     color: string;
  }
}

I use it only once, inside index.js.

Also a good place to add some global styles, if you need them. I use them for resetCSS (http://meyerweb.com/eric/tools/css/reset/) and some baseCSS rules like box-sizing etc.

index.js

import { createGlobalStyle, ThemeProvider } from 'styled-components';
import theme from './styles/theme';
import resetCSS from './styles/resetCSS';
import baseCSS from './styles/baseCSS';
import { BrowserRouter as Router} from "react-router-dom";

const GlobalStyle = createGlobalStyle`
  ${resetCSS}
  ${baseCSS}
`;

React.DOM.render(
  <React.Fragment>
    <GlobalStyle/>
    <Router>
      <ThemeProvider theme={theme}>
        <App/>
      </ThemeProvider>
    </Router>
  </React.Fragment>
  ,document.getElementById('root')
);