Create a global custom styles component

custom-styles-component.js

import { ThemeProvider, createGlobalStyle, css } from 'styled-components'

export const CustomStyles = createGlobalStyle`${props =>
  css`
    ${props.children};
  `}`;

// Usage
<ThemeProvider theme={theme}>
  {props.children}
  <CustomStyles>
    {/* user's custom styled go here */}
    {userTheme.customStyles}
  </CustomStyles>
</ThemeProvider>