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Theme UI follows the System UI Theme Specification, which defines standard scales and conventions for theme objects. This specification enables themes to be interoperable across different libraries and tools.

​
Theme Interface

The complete Theme type definition: 
export interface Theme {
  // Breakpoints for responsive styles
  breakpoints?: Array<string>
  mediaQueries?: { [size: string]: string }
  
  // Space scale for margin, padding, etc.
  space?: Scale<CSS.Property.Margin<number | string>>
  
  // Typography scales
  fonts?: Scale<CSS.Property.FontFamily>
  fontSizes?: Scale<CSS.Property.FontSize<number>>
  fontWeights?: Scale<CSS.Property.FontWeight>
  lineHeights?: Scale<CSS.Property.LineHeight<TLengthStyledSystem>>
  letterSpacings?: Scale<CSS.Property.LetterSpacing<TLengthStyledSystem>>
  
  // Sizing
  sizes?: Scale<CSS.Property.Height<{}> | CSS.Property.Width<{}>>
  
  // Border scales
  borders?: Scale<CSS.Property.Border<{}>>
  borderStyles?: Scale<CSS.Property.Border<{}>>
  borderWidths?: Scale<CSS.Property.BorderWidth<TLengthStyledSystem>>
  radii?: Scale<CSS.Property.BorderRadius<TLengthStyledSystem>>
  
  // Shadows and depth
  shadows?: Scale<CSS.Property.BoxShadow>
  zIndices?: Scale<CSS.Property.ZIndex>
  
  // Color scale (with color modes support)
  colors?: ColorModesScale
  rawColors?: ColorModesScale
  
  // Animation
  transitions?: Scale<CSS.Property.Transition>
  opacities?: Scale<CSS.Property.Opacity>
  
  // Text and color styles
  colorStyles?: Scale<ThemeUICSSProperties>
  textStyles?: Scale<ThemeUICSSProperties>
  
  // Configuration
  config?: ThemeUIConfig
  
  // Element styles
  styles?: ThemeStyles
  
  // Component variants
  buttons?: Record<string, ThemeUIStyleObject>
  text?: Record<string, ThemeUIStyleObject>
  links?: Record<string, ThemeUIStyleObject>
  images?: Record<string, ThemeUIStyleObject>
  cards?: Record<string, ThemeUIStyleObject>
  layout?: Record<string, ThemeUIStyleObject>
  forms?: Record<string, ThemeUIStyleObject>
  grids?: Record<string, ThemeUIStyleObject>
  badges?: Record<string, ThemeUIStyleObject>
  alerts?: Record<string, ThemeUIStyleObject>
  messages?: Record<string, ThemeUIStyleObject>
}

​
Scale Type

Scales can be defined as arrays or objects: 
export type Scale<T> = T[] | ScaleDict<T>

export interface ScaleDict<T> {
  [K: string]: T | T[] | NestedScaleDict<T> | undefined
  [I: number]: T
}

​
Required vs Optional Fields

All theme properties are optional. Theme UI provides sensible defaults: 
const defaultTheme = {
  space: [0, 4, 8, 16, 32, 64, 128, 256, 512],
  fontSizes: [12, 14, 16, 20, 24, 32, 48, 64, 72],
}

const defaultBreakpoints = [40, 52, 64].map(n => n + 'em')
// ['40em', '52em', '64em']

​
Color Modes Scale

The colors scale has special support for color modes: 
export interface ColorMode extends ScaleDict<CSS.Property.Color> {
  background?: ColorOrNestedColorScale
  text?: ColorOrNestedColorScale
  primary?: ColorOrNestedColorScale
  secondary?: ColorOrNestedColorScale
  accent?: ColorOrNestedColorScale
  highlight?: ColorOrNestedColorScale
  muted?: ColorOrNestedColorScale
}

export type ColorModesScale = ColorMode & {
  modes?: {
    [k: string]: ColorMode
  }
}
Example: 
const theme = {
  colors: {
    text: '#000',
    background: '#fff',
    primary: '#07c',
    secondary: '#609',
    modes: {
      dark: {
        text: '#fff',
        background: '#000',
        primary: '#0cf',
        secondary: '#f0f'
      }
    }
  }
}

​
Configuration Object

The config object controls Theme UI behavior: 
export interface ThemeUIConfig {
  /**
   * Enable/disable custom CSS properties/variables
   */
  useCustomProperties?: boolean

  /**
   * Initial color mode name
   */
  initialColorModeName?: string

  /**
   * Color mode for printing
   */
  printColorModeName?: string

  /**
   * Apply theme.styles.root to <html> element
   */
  useRootStyles?: boolean

  /**
   * Initialize color mode from prefers-color-scheme
   * 'system' | 'initial' | true | false
   */
  useColorSchemeMediaQuery?: 'system' | 'initial' | true | false

  /**
   * Add global box-sizing: border-box
   */
  useBorderBox?: boolean

  /**
   * Save color mode to localStorage
   */
  useLocalStorage?: boolean
}
Example: 
const theme = {
  config: {
    initialColorModeName: 'light',
    useColorSchemeMediaQuery: true,
    useLocalStorage: true,
    useRootStyles: true
  }
}

​
Theme Styles

The styles object defines base element styles: 
export interface ThemeStyles {
  root?: ThemeUIStyleObject     // Applied to <html> or <body>
  // Typography
  h1?: ThemeUIStyleObject
  h2?: ThemeUIStyleObject
  h3?: ThemeUIStyleObject
  h4?: ThemeUIStyleObject
  h5?: ThemeUIStyleObject
  h6?: ThemeUIStyleObject
  p?: ThemeUIStyleObject
  a?: ThemeUIStyleObject
  em?: ThemeUIStyleObject
  strong?: ThemeUIStyleObject
  // Code
  pre?: ThemeUIStyleObject
  code?: ThemeUIStyleObject
  inlineCode?: ThemeUIStyleObject
  // Lists
  ul?: ThemeUIStyleObject
  ol?: ThemeUIStyleObject
  li?: ThemeUIStyleObject
  // Other
  blockquote?: ThemeUIStyleObject
  hr?: ThemeUIStyleObject
  table?: ThemeUIStyleObject
  th?: ThemeUIStyleObject
  td?: ThemeUIStyleObject
  tr?: ThemeUIStyleObject
  img?: ThemeUIStyleObject
  // Custom
  [key: string]: ThemeUIStyleObject | undefined
}
Example: 
const theme = {
  styles: {
    root: {
      fontFamily: 'body',
      lineHeight: 'body',
      fontWeight: 'body',
    },
    h1: {
      fontSize: 5,
      fontFamily: 'heading',
      fontWeight: 'heading',
      lineHeight: 'heading',
      mt: 0,
      mb: 3
    },
    a: {
      color: 'primary',
      textDecoration: 'none',
      ':hover': {
        textDecoration: 'underline'
      }
    }
  }
}

​
Scale Conventions

​
Space Scale Convention

Use powers of two or a consistent rhythm: 
// Powers of two
space: [0, 4, 8, 16, 32, 64, 128, 256, 512]

// 4px rhythm
space: [0, 4, 8, 12, 16, 20, 24, 28, 32]

// Named scale
space: {
  none: 0,
  xs: 4,
  sm: 8,
  md: 16,
  lg: 32,
  xl: 64
}

​
Font Size Scale Convention

Define a typographic scale: 
// Modular scale
fontSizes: [12, 14, 16, 20, 24, 32, 48, 64, 72]

// Named scale
fontSizes: {
  xs: 12,
  sm: 14,
  base: 16,
  lg: 20,
  xl: 24,
  '2xl': 32,
  '3xl': 48
}

​
Color Scale Convention

Organize colors by purpose: 
colors: {
  // Base colors
  text: '#000',
  background: '#fff',
  
  // Brand colors
  primary: '#07c',
  secondary: '#609',
  accent: '#f80',
  
  // UI colors
  muted: '#f6f6f6',
  highlight: '#efeffe',
  
  // Feedback colors
  success: '#0a0',
  error: '#e00',
  warning: '#f80',
  info: '#07c',
  
  // Gray scale
  gray: ['#fafafa', '#f0f0f0', '#e0e0e0', '#ccc', '#999', '#666', '#333']
}

​
Nested Scales

Nested scales allow grouping related values: 
const theme = {
  colors: {
    primary: {
      __default: '#00f',
      light: '#33f',
      dark: '#00a'
    },
    gray: {
      __default: '#666',
      light: '#ccc',
      dark: '#333'
    }
  }
}
The __default key is used when accessing the parent: 
<Box sx={{ color: 'primary' }} />
// Uses primary.__default: #00f

<Box sx={{ color: 'primary.light' }} />
// Uses primary.light: #33f

​
Component Variants

Component variants are reusable style objects: 
const theme = {
  buttons: {
    primary: {
      color: 'white',
      bg: 'primary',
      fontWeight: 'bold',
      borderRadius: 2,
      cursor: 'pointer',
      '&:hover': {
        bg: 'primaryHover'
      }
    },
    secondary: {
      color: 'text',
      bg: 'muted',
      border: '1px solid',
      borderColor: 'gray.3'
    },
    ghost: {
      color: 'primary',
      bg: 'transparent',
      border: '1px solid',
      borderColor: 'primary'
    }
  },
  text: {
    heading: {
      fontFamily: 'heading',
      fontWeight: 'heading',
      lineHeight: 'heading'
    },
    caps: {
      textTransform: 'uppercase',
      letterSpacing: '0.1em'
    }
  }
}

​
Breakpoints

Breakpoints define responsive design boundaries: 
// Array format (mobile-first)
breakpoints: ['40em', '52em', '64em']

// Named breakpoints
breakpoints: ['640px', '768px', '1024px', '1280px']
Use with responsive arrays: 
<Box sx={{ fontSize: [1, 2, 3, 4] }} />
// fontSize: 14px (mobile)
// fontSize: 16px (> 640px)
// fontSize: 20px (> 768px)
// fontSize: 24px (> 1024px)
Following the Theme Specification ensures your themes work consistently across different Theme UI applications and can be shared or extended easily.

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