Substack UI Skills

Opinionated constraints for building Substack-style interfaces with AI agents.

When to Apply

Reference these guidelines when:

• Building light-mode interfaces
• Creating Substack-inspired design systems
• Implementing UIs with Inter font and 4px grid

Colors

• SHOULD use light backgrounds for primary surfaces
• MUST use #FFFFFF as page background (surface-base)
• SHOULD reduce color palette (currently 15 colors detected)
• MUST maintain text contrast ratio of at least 4.5:1 for accessibility

Semantic Tokens

Typography

• MUST use Inter as primary font family
• SHOULD use single font family for consistency
• MUST use 36px / semi_bold for primary headings
• MUST use 17px / 500 for body text
• SHOULD reduce font weights (currently 4 detected)
• MUST use text-balance for headings and text-pretty for body text
• SHOULD use tabular-nums for numeric data
• NEVER modify letter-spacing unless explicitly requested

Text Styles

Typography Reference

Font Families:

• Inter (used 43x)

Font Sizes: 5px, 9px, 10px, 11px, 12px, 13px, 14px, 15px, 16px, 17px, 19px, 36px

Spacing

• MUST use 4px grid for spacing
• SHOULD use spacing from scale: 1px, 3px, 4px, 5px, 8px, 9px, 10px, 13px
• SHOULD use 4px as default gap between elements
• NEVER use arbitrary spacing values (use design scale)
• SHOULD maintain consistent padding within containers

Borders

• MUST use border-radius from scale: 3px, 4px, 6px, 7px, 8px, 9px, 16px, 18px
• MUST use 1px border width consistently
• SHOULD use 9px as default border-radius
• NEVER use arbitrary border-radius values (use design scale)
• SHOULD use subtle borders (1px) for element separation

Border Radius Reference

Scale: 3px, 4px, 6px, 7px, 8px, 9px, 16px, 18px

Layout

• MUST design for 1920px base viewport width
• SHOULD use consistent element widths: 12px, 16px, 18px, 17px, 22px
• SHOULD maintain text-heavy layout with clear hierarchy
• NEVER use h-screen, use h-dvh for full viewport height
• MUST respect safe-area-inset for fixed elements
• SHOULD use size-* for square elements instead of w-* + h-*

Detected Layout Patterns

• Main Content: width: 1920px, height: 1080px

Components

• MUST use 0px height for input fields

Buttons

Inputs

• Height: 0px

Interactive States

Focus

• MUST use 2px outline with accent color (#5E6AD2)
• MUST use 2px outline-offset
• NEVER remove focus indicators

Hover

• Buttons (primary): lighten background by 10%
• Buttons (secondary): use #FC5013 background
• List items: use #FC5013 background

Disabled

• MUST use opacity: 0.5
• MUST use cursor: not-allowed

Interaction

• MUST use an AlertDialog for destructive or irreversible actions
• SHOULD use structural skeletons for loading states
• MUST show errors next to where the action happens
• NEVER block paste in input or textarea elements
• MUST add an aria-label to icon-only buttons

Animation

• NEVER add animation unless it is explicitly requested
• MUST animate only compositor props (transform, opacity)
• NEVER animate layout properties (width, height, top, left, margin, padding)
• SHOULD use ease-out on entrance animations
• NEVER exceed 200ms for interaction feedback
• SHOULD respect prefers-reduced-motion

Performance

• NEVER animate large blur() or backdrop-filter surfaces
• NEVER apply will-change outside an active animation
• NEVER use useEffect for anything that can be expressed as render logic