How to style body tag with CSS-in-JS approach?

With Emotion you can set something up, like the following create-react-app example, to inject global styles:

import React from 'react';import ReactDOM from 'react-dom';import { Global, css } from '@emotion/core'const bodyFillColor = `rgb(218,236,236)`;class App extends React.Component {  render() {    return(      <div>        <Global          styles={css`            body {              background: ${bodyFillColor};              margin: 0;              padding: 0;              min-height: '100vh';              max-width: '100vw';            }          `}        />        <Global          styles={{            'body.noScroll': {                // Prevent scrolling; conditionally activate this                // in subcomponents when necessary ...                overflow: 'hidden',            },          }}        />      </div>    );  }}ReactDOM.render(  <App />,  document.getElementById('root'));

This shows an example of injecting a style on the body and also assigning a class to the body that can conditionally be activated later on.

eg.

{this.state.activate && <Global styles={{`stylesetc`}}/>}

https://emotion.sh/docs/globals

Alternative

StyledComponents uses a CSS-in-JS approach and works great with React applications. This is a technique I've used in the past straight from the documentation:

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>

If you're using react application you can create index.css file and set your wanted properties for the body. Then you must import the index.css file in your index.js file and the changes will take place.

As per the question if the task is as small as changing body's background color in js then below approach can also be followed any where in your code most probably in App.js

if(theme==='dark')  document.body.style.background = '#000'else  document.body.style.background = '#FFF' 

No need to use a whole styling library for it.

Also i tried editing document.body.style, you can try that too according to below example

if(theme==='dark')  bgColor = '#000'else  bgColor = '#FFF'document.body.style= `background: ${bgColor}`

Remember following 2nd approach you may overwrite whole body style so please take care of that.

I hope this helps :)