Motivation

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

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 with the 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 coming years.

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. Using the 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 includes the 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.

The css and sx Props

The css prop is a powerful pattern for styling UI in React. It works like the built-in 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 native JavaScript object literal syntax. The 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 css prop also avoids some of the pitfalls of mixing CSS properties with HTML attributes.

The 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 global theme object.

Why Object Literal Syntax

For many, the choice between using object literal syntax for styles versus tagged template literals comes down to personal preference, but in the case of Theme UI, there are some fundamental reasons for using native JavaScript types for styles.

The 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 syntax. Additionally, using native JavaScript types has many other benefits that are outside of the scope of this document.

Why Emotion

While there are many different solutions to handling CSS in JavaScript, Styled Components and Emotion have become the most widely-used industry-standard libraries. If you're building a custom component library, either Styled Components or Emotion should suit your needs just fine.

For Theme UI, the decision was primarily based on these factors:

• Emotion's implementation of the css prop and the custom JSX pragma allows for better integration with Theme UI's sx prop
• The Emotion API includes more lower-level utilities, like createEmotion that 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 styled higher-order component utility is not necessarily the best API for creating components, and by not including @emotion/styled in the core package the bundle size is kept to a minimum – i.e. most of the same things can be achieved with the css prop

How is this different from Styled System

Theme UI's 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.