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From the very beginning, React UI has been designed with a great emphasis on customizability. We decided to leverage CSS custom properties for this feature for two main reasons:

  1. We take advantage of the possibilities of native CSS. Preprocessors are still a thing, but it's not necessary to go as far as for CSS-in-JS to make a UI customizable, not even speaking of performance.

  2. Thanks to JavaScript API, CSS custom properties are both readable and writable by JS.

Theming Options

Design tokens define common visual properties like colors, fonts, borders, shadows, or spacing. CSS custom properties are the technical representation of the design tokens in React UI.

All CSS custom properties in React UI come prefixed with rui so they don't get in the way of other custom properties in your project.

You can adjust any of the properties in your styles. See the default theme for the full list of available design tokens.

Global and Semantic Design Tokens

Global and semantic token names are not complex or long. That is why they are simply lowercase and hyphenated.

The names are written in the following format:



  • <type> is one of: color, dimension, font-family, font-weight, shadow, as suggested by the Design Tokens Format proposal. However, additional custom types like font-size, line-height, or text-decoration have been added as they proved necessary.
  • <group> optionally groups multiple related values, e.g. text, background, action, etc.
  • <name> is the name of the token, e.g. primary, base, or light. Scales can be presented as numbered sequences, e.g. space-[0-7], size-[1-6], etc.
  • <state> describes additional interaction variants of the token: hover, focus, active, or disabled.

Example global and semantic design tokens represented by CSS custom properties:

:root {
  --rui-color-text-primary: #000;
  --rui-dimension-space-3: 0.75rem;
  --rui-font-family-base: "Titillium Web", helvetica, roboto, arial, sans-serif;

️👉 Please note that breakpoint values are read-only (e.g. for JavaScript) since CSS custom properties cannot be used within media queries (because a media query is not a CSS property).

Component Tokens

It is also possible to adjust some properties on individual components level, preferably by reusing (inheriting) the semantic design tokens.

Due to higher complexity, component tokens use a naming convention that many web developers will find familiar because it works like BEM (with prefixes and component name syntax taken from SUIT CSS, to be precise):



  • <ComponentName> stands for the actual component name (e.g. Button, FormField, etc.) with a reasonable exception to form fields whose settings are widely shared and therefore grouped as FormField options.
  • <modifications(s)> can be one or more modifiers, typically a variant (e.g. primary, filled, box) or interaction state (default, hover, focus, active, disabled).
  • <element> stands for a nested element of the component.
  • <property> is usually a CSS property (e.g. color, background, background-color, width, box-shadow), or a brief property description where a CSS property wouldn't tell enough (e.g. initial-offset, check-background-color, tap-target-size).

Example component tokens:

:root {
  --rui-Button--filled--primary--default__color: var(--rui-color-action-on-primary);
  --rui-Button--filled--primary--default__border-color: var(--rui-color-action-primary);
  --rui-Button--filled--primary--default__background: var(--rui-color-action-primary);
  --rui-Button--filled--primary--default__box-shadow: none;
  --rui-Button--filled--primary--hover__color: var(--rui-color-action-on-primary);
  --rui-Button--filled--primary--hover__border-color: var(--rui-color-action-primary-hover);
  --rui-Button--filled--primary--hover__background: var(--rui-color-action-primary-hover);
  --rui-Button--filled--primary--hover__box-shadow: none;

Best Practices

  1. It's a good idea to start with changing the global tokens first. Adjust any context-agnostic values to see how the system reacts and scales.

  2. Widely reused context-aware settings such as semantic colors, typography, or borders define the character of your design system which is stored in the semantic tokens layer.

  3. Having finished the customization at the global and semantic level, you can then proceed to customize the appearance of individual components — if necessary at all.

Even then you should also reuse existing semantic design tokens as much as possible to ensure that your UI is consistent and works as a system.

For the same reason, if you have any custom components in your UI, you should reuse the semantic design tokens in your own CSS too.

CSS, or Sass?

Colors, breakpoints, and SVG definitions used in theme.scss are preprocessed with Sass first. This enables us to:

  • generate our internal color palette programmatically,
  • keep actual breakpoint values in a single place in the code,
  • keep theme.scss uncluttered by inline SVG.

It's entirely up to you what format you decide to use for storing the theme. Both theme.scss and theme.css will work equally well. It only matters if the custom properties make it from the theme file to the browser.

👉 Just remember everything in the theme constants directory is intended only for usage within theme.scss. Otherwise, the theming system may not work as expected. We recommend calling custom properties from theme.scss either directly in your stylesheet, or through an intermediate, shareable layer like MyComponent/_theme.scss or styles/shared-by-components/_my-sass-variables-referring-to-theme.scss (the latter of which is the approach we use).