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

Token HEX RGB Usage

surface-base

#FFFFFF rgb(255,255,255) Page background

surface-raised

#FC5013 rgb(252,80,19) Cards, modals, raised surfaces

surface-overlay

#E9E9E9 rgb(233,233,233) Overlays, tooltips, dropdowns

text-primary

#999999 rgb(153,153,153) Headings, body text

text-2

#515151 rgb(81,81,81) Additional text

text-secondary

#717171 rgb(113,113,113) Secondary, muted text

border-default

#E7E7E7 rgb(231,231,231) Subtle borders, dividers

destructive

#E26739 rgb(226,103,57) Error states, delete actions

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

Style Font Size Weight Color Count

heading-1

Inter 36px semi_bold #BCDBD7 1

text-19px

Inter 19px semi_bold #4C4D4D 1

body

Inter 17px 500 #535353 1

body-secondary

Inter 16px 400 #717171 1

body-secondary

Inter 16px 400 #6E6E6F 1

body-secondary

Inter 16px 400 #F5C0A3 1

body-secondary

Inter 15px 400 #969696 2

body-secondary

Inter 15px 500 #515151 1

body-secondary

Inter 15px 400 #6F6F6F 1

body-secondary

Inter 15px 500 #5D5D5D 1

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

Variant Background Text Border Height Radius

Ghost transparent #515151 none

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