System UI Theme Specification

https://system-ui.com

This specification is a work-in-progress. Please see the related issue for more.

The theme object is intended to be a general purpose format for storing design system style values, scales, and/or design tokens. The object itself is not coupled to any particular library's implementation and can be used in places where sharing common style values in multiple parts of a code base is desirable.

Scale Objects

Many CSS style properties accept open-ended values like lengths, colors, and font names. In order to create a consistent styling system, the theme object is centered around the idea of scales, such as a typographic (font-size) scale, a spacing scale for margin and padding, and a color object. These scales can be defined in multiple ways depending on needs, but tend to use arrays for ordinal values like font sizes, or plain objects for named values like colors, with the option of nesting objects for more complex systems.

// example fontSizes scale as an array
fontSizes: [
  12, 14, 16, 20, 24, 32
]

// example colors object
colors: {
  blue: '#07c',
  green: '#0fa',
}

// example nested colors object
colors: {
  blue: '#07c',
  blues: [
    '#004170',
    '#006fbe',
    '#2d8fd5',
    '#5aa7de',
  ]
}

Scale Aliases

For typically ordinal values like font sizes that are stored in arrays, it can be helpful to create aliases by adding named properties to the object.

// example fontSizes aliases
fontSizes: [
  12, 14, 16, 20, 24, 32
]
// aliases
fontSizes.body = fontSizes[2]
fontSizes.display = fontSizes[5]

Excluded Values

Some CSS properties accept only a small, finite number of valid CSS values and should not be included as a scale object. For example, the text-align property accepts the following values: left, right, center, justify, justify-all, start, end, or match-parent. Other properties that are intentionally excluded from this specification include: float, clear, display, overflow, position, vertical-align, align-items, justify-content, and flex-direction.

Keys

The keys in the theme object should typically correspond with the CSS properties they are used for, and follow a plural naming convention. For example, the CSS property font-size is expected to use values from the fontSizes scale, and the color property uses values from the colors scale.

Some keys can be used for multiple CSS properties, where the data type is the same. The color object is intended to be used with any property that accepts a CSS color value, such as background-color or border-color.

Space

The space key is a specially-named scale intended for use with margin, padding, and other layout-related CSS properties. A space scale can be defined as either a plain object or an array, but by convention an array is preferred. This is an intentional constraint that makes it difficult to add "one-off" or "in-between" sizes that could lead to unwanted and rippling affects to layout.

Note: other names under consideration include spacing, spaces, and lengths.

When defining space scales as an array, it is conventional to use the value 0 as the first value so that space[0] === 0.

// example space scale
space: [
  0, 4, 8, 16, 32, 64
]

// example space scale object
space: {
  small: 4,
  medium: 8,
  large: 16,
}

// example space scale with aliases
space: [
  0, 4, 8, 16, 32
]
space.small = space[1]
space.medium = space[2]
space.large = space[3]

Breakpoints

Breakpoints are CSS lengths intended for use in media queries. Commonly, the breakpoints scale is used to create mobile-first responsive media queries based on array values.

Media Queries

For convenience and for use with other styling approaches, a mediaQueries scale derived from the breakpoints scale can be added to the theme object.

breakpoints: [ '40em', '52em', '64em' ]

mediaQueries: {
  small: `@media screen and (min-width: ${breakpoints[0]})`,
  medium: `@media screen and (min-width: ${breakpoints[1]})`,
  large: `@media screen and (min-width: ${breakpoints[2]})`,
}

Key Reference

The following is a list of theme object keys and their corresponding CSS properties. This list may be non-exhaustive.

Theme Key | CSS Properties ------------------|-------------- space | margin, margin-top, margin-right, margin-bottom, margin-left, padding, padding-top, padding-right, padding-bottom, padding-left, grid-gap, grid-column-gap, grid-row-gap fontSizes | font-size colors | color, background-color, border-color fonts | font-family fontWeights | font-weight lineHeights | line-height letterSpacings | letter-spacing sizes | width, height, min-width, max-width, min-height, max-height borders | border, border-top, border-right, border-bottom, border-left borderWidths | border-width borderStyles | border-style radii | border-radius shadows | box-shadow, text-shadow zIndices | z-index transitions | transition

Prior Art

Prior art includes, but is not limited to the following. Please open an issue or pull request to help ensure this list is inclusive.

• Theo
• Style Dictionary
• Lona
• Universal Design Tokens
• Styled System