One of the primary motivations behind Theme UI is to make building themeable, constraint-based user interfaces in React as simple and as interoperable as possible. This library builds upon years of work on various libraries, including Basscss, Tachyons, Rebass, Styled System, and various other internal style guides and design systems. While some of the ideas encapsulated within this library may seem familiar, the intent here is to combine modern CSS tooling into a single "mini-framework" with good guidance on how to fall into the pit of success for creating white-label products, themed component libraries, and even full websites.
MDX has, in my opinion, quickly become one of the best ways to render Markdown
in React. The ability to render custom React components for any Markdown element
MDXProvider is a very powerful API and has the potential to open up
how Markdown is leveraged in ways I think we'll continue to see evolve over the
While the final rendered HTML can be styled using global CSS or a wrapping
element with child selectors, there are certainly drawbacks to this approach,
and they can lead to unexpected styling bugs when composing themes together.
MDXProvider to render custom styled components in MDX is a great way
to avoid this, but the overhead for applying styles in this way can be a lot of
work, even with UI component libraries like Rebass or Material UI. Theme UI
theme.styles API as a light abstraction on top of this, that
hopefully feels familiar to people from diverse backgrounds, even those with
little or no experience using MDX.
For examples of previous explorations into this idea, see Rebass MDX, MDX Style, and MDX Blocks.
css prop is a powerful pattern for styling UI in React. It works like the
style prop, but it includes some of the best parts of the CSS
language, including media queries and pseudo-classes, and can be authored in
css prop can be leveraged in a
similar way to the
styled higher-order component, but also offers more
flexibility when making one-off, context-specific stylistic changes. The
prop also avoids some of the
pitfalls of mixing CSS properties with HTML attributes.
sx prop is a light abstraction on top of the
css prop that can serve as
a complete replacement and makes it easier to ensure you use values from your
Why Object Literal Syntax
theme object itself is an object, and keeping styles in a similar format
helps reduce the API surface area. Using and parsing strings that represent
embedded DSLs introduces overhead when mapping over key-value pairs. Theme UI
avoids this overhead for reasons related to performance, testing, and overall
bundle size. For some of the same reasons
that React itself uses JSX (i.e. function calls) instead of tagged template
literals, Theme UI only includes support for authoring CSS with object literal
are outside of the scope of this document.
For Theme UI, the decision was primarily based on these factors:
- Emotion's implementation of the
cssprop and the custom JSX pragma allows for better integration with Theme UI's
- The Emotion API includes more lower-level utilities, like
createEmotionthat could be leveraged when considering how multiple themes could be composed together
- Emotion's theming context is directly available in
@emotion/react, allowing this library to leverage React's context API in different ways
- In the case of Theme UI internals, the
styledhigher-order component utility is not necessarily the best API for creating components, and by not including
@emotion/styledin the core package the bundle size is kept to a minimum – i.e. most of the same things can be achieved with the
How is this different from Styled System
sx prop was inspired by Styled System, and it uses the same theme
spec that Styled System adheres to. Styled System is a much lower-level API that
is not in any way coupled to React or Emotion. For example, Styled System works
with Node.js, Vue, Svelte, and many other libraries. Theme UI is intended to be
a higher-level abstraction specifically for use in React applications.