styled-components - how to set styles on html or body tag?

You can, of course, maintain a separate CSS file that you include in your HTML via a <link> tag.

For v4:

Use createGlobalStyle from Styled-components.

import { createGlobalStyle } from 'styled-components'

const GlobalStyle = createGlobalStyle`
  body {
    color: ${props => (props.whiteColor ? 'white' : 'black')};
  }
`

<React.Fragment>
  <GlobalStyle whiteColor />
  <Navigation /> {/* example of other top-level stuff */}
</React.Fragment>

Pre v4:

Styled-components also exports an injectGlobal helper to inject global CSS from JavaScript:

import { injectGlobal } from 'styled-components';

injectGlobal`
  html {
    height: 100%;
    width: 100%;
  }
`

See the API documentation for more information!

As of Oct, 2018.

NOTE

The injectGlobal API was removed and replaced by createGlobalStyle in styled-components v4.

According to docs createGlobalStyle is

A helper function to generate a special StyledComponent that handles global styles. Normally, styled components are automatically scoped to a local CSS class and therefore isolated from other components. In the case of createGlobalStyle, this limitation is removed and things like CSS resets or base stylesheets can be applied.

Example

import { createGlobalStyle } from 'styled-components'

const GlobalStyle = createGlobalStyle`
  body {
    color: ${props => (props.whiteColor ? 'white' : 'black')};
  }
`

// later in your app

<React.Fragment>
  <Navigation /> {/* example of other top-level stuff */}
  <GlobalStyle whiteColor />
</React.Fragment>

Read more on official docs.