Build consistent, themeable React UIs based on design system constraints and design tokens

https://theme-ui.com

• Style your application consistently with a global theme object and customizable design tokens
• Style MDX content with themes
• Follows the Theme Specification for interoperability with other libraries
• Style elements directly with the css prop and @styled-system/css
• Works with or without custom styled components and Styled System
• First-class support for Typography.js themes
• First-class support for dark mode and other color modes
• Use contextual theming for nested stylistic changes
• Primitive page layout components
• Keep styles isolated with Emotion

npm i theme-ui @emotion/core @emotion/styled @mdx-js/react

Wrap your application with the ThemeProvider component and pass in a custom theme object.

// basic usage
import React from 'react'
import { ThemeProvider } from 'theme-ui'
import theme from './theme'

export default props =>
  <ThemeProvider theme={theme}>
    {props.children}
  </ThemeProvider>

The theme object should follow the System UI Theme Specification, which lets you define custom color palettes, typographic scales, fonts, and more.

// example theme.js
export default {
  fonts: {
    body: 'system-ui, sans-serif',
    heading: '"Avenir Next", sans-serif',
    monospace: 'Menlo, monospace',
  },
  colors: {
    text: '#000',
    background: '#fff',
    primary: '#33e',
  },
}

Use the css prop throughout your application, along with the css utility to pick up values from the theme. Using the css utility means that color and other values can reference values defined in theme object.

import React from 'react'
import { css } from 'theme-ui'

export default () =>
  <div
    css={css({
      fontWeight: 'bold',
      fontSize: 4,  // picks up value from `theme.fontSizes[4]`
      color: 'primary', // picks up value from `theme.colors.primary`
    })}>
    Hello
  </div>

Read more about the css utility in the @styled-system/css docs.

The css utility also supports using arrays as values to change properties responsively with a mobile-first approach.

import React from 'react'
import { css } from 'theme-ui'

export default props =>
  <div
    css={css({
      // applies width 100% to all viewport widths,
      // width 50% above the first breakpoint,
      // and 25% above the next breakpoint
      width: [ '100%', '50%', '25%' ],
    })}
  />

To use values from the theme object without importing the css utility, use the custom jsx pragma, which will automatically convert Theme UI css prop values.

/** @jsx jsx */
import { jsx } from 'theme-ui'

export default props =>
  <div
    css={{
      fontSize: 4,
      color: 'primary',
      bg: 'lightgray',
    }}
  />

Read more in the custom pragma docs.

Use the components prop to add components to MDX scope. This can be used to introduce new behavior to markdown elements in MDX, such as adding linked headings or live-editable code examples. Internally, the ThemeProvider is a combination of MDXProvider and Emotion's ThemeProvider.

// with mdx components
import React from 'react'
import { ThemeProvider } from 'theme-ui'
import mdxComponents from './mdx-components'
import theme from './theme'

export default props =>
  <ThemeProvider
    components={mdxComponents}
    theme={theme}>
    {props.children}
  </ThemeProvider>

The MDX components can also be styled via the theme.styles object. This can be used as a mechanism to pass in fully-baked themes and typographic styles to MDX content.

// example theme
export default {
  colors: {
    primary: '#33e',
  },
  styles: {
    // this styles child MDX `<h1>` components
    h1: {
      fontSize: 32,
      // this value comes from the `color` object
      color: 'primary',
    },
  }
}

These components' styles can also be consumed outside of an MDX doc with the Styled component. Note that these are only styled using the same theme.styles object and not the same components passed to the ThemeProvider context.

import React from 'react'
import { Styled } from 'theme-ui'

export default props =>
  <Styled.div>
    <Styled.h1>
      Hello
    </Styled.h1>
  </Styled.div>

To change the underlying component in Styled, use the as prop.

<Styled.a as={Link} to='/'>Home</Styled.a>

Theme UI includes several components for creating page layouts.

• Layout: sets a flex column with 100vh min-height
• Header: flexbox row
• Footer: flexbox row
• Main: flex auto container for pushing the Footer to the bottom of the viewport
• Container: max-width, centered container

import React from 'react'
import {
  Layout,
  Header,
  Main,
  Container,
  Footer
} from 'theme-ui'

export default props =>
  <Layout>
    <Header>
      Hello
    </Header>
    <Main>
      <Container>
        {props.children}
      </Container>
    </Main>
    <Footer>
      © 2019
    </Footer>
  </Layout>

The Box & Flex layout components are similar to the ones found in Rebass, but are built with Emotion and @styled-system/css.

import React from 'react'
import { Flex, Box } from 'theme-ui'

export default props =>
  <Flex flexWrap='wrap'>
    <Box width={[ 1, 1/2 ]} />
    <Box width={[ 1, 1/2 ]} />
  </Flex>

• Theming
• Layout
• Color Modes
• Gatsby Plugin
• Custom Pragma
• Typography.js

MIT License