Tailwind

units

Always use Tailwind units first. For custom values, always use rem, never px.

// BAD - tailwind units available but custom rem used
return <div className="p-[1.0625rem]" />;

// GOOD - uses tailwind units
return <div className="p-4.25" />;

tailwind-merge

Use tailwind-merge to concatenate class names in React components instead of template literals or array joins.

• `twJoin`: for conditional class combinations (no override needed)
• `twMerge`: allow class override (merges conflicting utilities; perfect for setting default and optional classes)

Examples

import {twMerge, twJoin} from 'tailwind-merge';

// BAD, template literals with potential conflicts
return <span className={`bg-gray-500 ${isBlue ? 'bg-blue-500' : ''}`} />;

// GOOD - twJoin for conditional classes, but no override needed
return <span className={twJoin('bg-gray-800', isBlue && 'text-blue-500')} />;

// GOOD, twMerge for override, twJoin for conditional classes
return <span className={twMerge('text-white', isBlue && 'text-blue-500')} />;

The key distinction: use twMerge when a component accepts a className prop that should be able to override defaults, twJoin would leave both conflicting classes on the element, but twMerge resolves the conflict:

// twMerge enables callers to override component defaults
function Button({className}: {className?: string}) {
  return (
    <button
      className={twMerge('bg-blue-500 px-4 py-2 text-white', className)}
    />
  );
}

// bg-red-500 wins, twMerge removes the conflicting bg-blue-500
<Button className="bg-red-500" />;

Conditional classes

Pass falsy values directly to twJoin / twMerge, they're skipped.

// correct
twJoin('base', isActive && 'bg-blue-500', error && 'border-red-500');

// ternaries that produce two positive values are fine
twJoin('base', condition ? 'a' : 'b');

Don't wrap class lists in template literals to concatenate, pass each class as a separate argument. Template literals inside twJoin / twMerge are acceptable only when interpolating a pre-built string from a lookup table (e.g., ICON_POSITION[iconPosition] from a Record<string, string>).

Variant / size lookup tables

Extract multi-class variant strings into Record constants at the top of the file, then reference them positionally, not via interpolation.

const VARIANTS: Record<Variant, string> = {
  primary: 'border border-blue-400 bg-blue-500 text-white ...',
  secondary: '...',
};

// usage
twJoin('rounded-sm px-3 py-2', VARIANTS[variant]);

Custom Values

When Tailwind's built-in scale doesn't have an exact value, use an arbitrary value with rem, never px:

// BAD, px unit
<p className="text-[9px]" />

// GOOD, rem unit
<p className="text-[0.5625rem]" />

Use arbitrary values sparingly. If the same custom value appears more than once, add it to the tailwind.css @theme instead.

Custom Theme Tokens

When adding @theme tokens to app/styles/tailwind.css, name them by role, not by the utility they'll be used with. The CSS variable suffix becomes the suffix on every generated utility (bg-, text-, border-, ring-, fill-, stroke-, from-, to-, etc.), so a token containing a utility prefix produces stuttering class names.

Lint check before naming: mentally expand bg-{name}, text-{name}, border-{name}. If any reads as a stutter or nonsense, rename.

/* BAD, produces bg-bg, text-bg, border-bg-tint, text-text-muted */
--color-bg: #141413;
--color-bg-tint: #181c1e;
--color-text: #e0e0e0;
--color-text-muted: #999;

/* GOOD, produces bg-canvas, bg-surface, text-ink, text-muted */
--color-canvas: #141413;
--color-surface: #181c1e;
--color-ink: #e0e0e0;
--color-muted: #999;

Role vocabulary that survives the lint check: canvas, surface, surface-raised, ink, muted, subtle, accent, brand-*, success, warning, danger, palette-style names like claude-500. Avoid token names starting with bg-, text-, border-, ring-, fill-, stroke-, from-, to-, outline-, shadow-.

For paired light/dark colors, prefer defining an @utility (like the existing bg-body, text-body) over a single @theme token, @utility binds two palette values together; a @theme token exposes one hex on every utility prefix.