Backpack CSS Vars Migration

Backpack CSS Vars Migration

Problem

Backpack components use static SASS tokens (e.g. tokens.$bpk-text-primary-day) that are hardcoded to light-mode values. The new CSS var system (theme-backpack-light.css / theme-backpack-dark.css / primitives.css) defines custom properties that flip automatically with the active theme, but components need to opt in by using var().

Context / Trigger Conditions

• Component SCSS uses bare tokens.$bpk-*-day values with no var() wrapping
• Component has a mixin file using @include utils.bpk-themeable-property(...) with old --bpk-{component}-* var names
• Stories have themed examples that no longer apply overrides correctly
• themeAttributes.tsx lists camelCase keys that map to old var names not present in SCSS

Key Files

Solution

Step 1: Discover available CSS vars

Check what vars exist for the component across all three CSS files:

grep -i "{component}" packages/backpack-web/src/bpk-stylesheets/theme-backpack-light.css
grep -i "{component}" packages/backpack-web/src/bpk-stylesheets/primitives.css

Also check global semantic vars that may apply (colours, spacing, radii):

grep -E "bpk-text-|bpk-core-|bpk-status-|bpk-surface-" packages/backpack-web/src/bpk-stylesheets/theme-backpack-light.css
grep -E "bpk-spacing-|bpk-radius-|bpk-border-" packages/backpack-web/src/bpk-stylesheets/primitives.css

Step 2: Migrate the SCSS mixin

Replace every bare token and bpk-themeable-property call with a plain var():

// Before
@include utils.bpk-themeable-property(
  background-color,
  --bpk-badge-normal-background-color,
  tokens.$bpk-private-badge-background-normal-day
);

// After
background-color: var(--bpk-private-badge-colour-bg-default, tokens.$bpk-private-badge-background-normal-day);

Pattern: property: var(--new-css-var, tokens.$sass-fallback);

No nested fallbacks. No cascade chains. One CSS var, one SASS fallback.

Colour properties

Map each token to its CSS var equivalent:

Spacing / dimensions

// Use primitive vars with SASS fallbacks
padding: var(--bpk-spacing-sm, tokens.bpk-spacing-sm()) var(--bpk-spacing-md, tokens.bpk-spacing-md());

// Or use a component-specific dimension var if one exists:
padding: var(--bpk-spacing-sm, tokens.bpk-spacing-sm()) var(--bpk-private-{component}-dimension-padding-horizontal-default, tokens.bpk-spacing-md());

Border radius

// Use the primitive, not a component-specific var
border-radius: var(--bpk-radius-xs, tokens.$bpk-border-radius-xs);
// Other sizes: --bpk-radius-sm (0.5rem), --bpk-radius-md (0.75rem), --bpk-radius-lg (1.5rem)

Border width

Match by pixel value to the primitive:

// $bpk-border-size-sm = 1px = 0.0625rem = --bpk-border-1
border: var(--bpk-border-1, tokens.$bpk-border-size-sm) solid ...;
// Other sizes: --bpk-border-2 (0.125rem), --bpk-border-3 (0.1875rem)

Typography

Replace @include typography.bpk-footnote (and similar) with explicit declarations so each property can have its own var.

Check for --bpk-private-{component}-typography-tmp-* vars first — these are transitional names that exist in theme-backpack-light/dark.css but will be removed. If one exists, use it with a SASS fallback. If not (or if the var is marked tmp and removal is planned), use the SASS token directly — no CSS var wrapper.

// When a stable CSS var exists:
font-size: var(--bpk-private-{component}-typography-stable-name, tokens.$bpk-font-size-sm);

// When only a tmp var exists (will be removed) — use SASS token directly:
font-size: tokens.$bpk-font-size-xs;

// Font weight / line-height (stable vars):
font-weight: var(--bpk-{component}-font-weight, tokens.$bpk-font-weight-book);
line-height: var(--bpk-{component}-line-height, tokens.$bpk-line-height-sm);

When dropping a tmp font-size var, also remove the corresponding camelCase key (e.g. badgeFontSize) from themeAttributes and its test — it's no longer themeable via that route.

Typography mixins bpk-text and bpk-caption: bpk-text only sets margin: 0 (already in the base mixin). bpk-caption only sets font-size, line-height, and font-weight via _bpk-text-factory — no font-family or letter-spacing. Our three explicit var() declarations fully replace them.

RTL / logical properties

Replace physical properties + bpk-rtl mixin with CSS logical properties:

// Before
right: 0;
@include utils.bpk-rtl { right: inherit; left: 0; }

// After
inset-inline-end: 0;

top    → block-start        bottom → block-end
left   → inline-start       right  → inline-end
border-top-left-radius     → border-start-start-radius
border-top-right-radius    → border-start-end-radius
border-bottom-left-radius  → border-end-start-radius
border-bottom-right-radius → border-end-end-radius

Step 3: Clean up unused imports

After migrating, remove any @use imports that are no longer referenced:

grep "utils\.\|typography\.\|radii\." packages/backpack-web/src/bpk-mixins/_{component}.scss

Step 4: Update themeAttributes.tsx

BpkThemeProvider converts camelCase keys → --bpk-{kebab-case} inline styles. Inline styles have higher specificity than stylesheets, so themed overrides win automatically.

Only expose --bpk-private-{component}-* vars as themeable keys. Global semantic vars (--bpk-text-primary, --bpk-core-primary, --bpk-radius-xs, etc.) are theme-level concerns — they apply across all components and should be set globally, not per-component in a badge's themeAttributes.

Update per-variant arrays to only list private component-scoped keys:

// Key → CSS var injected by BpkThemeProvider
// privateBadgeColourBgDefault → --bpk-private-badge-colour-bg-default

// ✓ Correct — component-scoped private var
export const badgeNormalThemeAttributes = ['privateBadgeColourBgDefault'];

// ✗ Wrong — global semantic var; override at theme level, not here
export const badgeNormalThemeAttributes = ['privateBadgeColourBgDefault', 'textPrimary'];

Camelcase conversion rule: strip --bpk-, convert kebab to camelCase. e.g. --bpk-private-badge-colour-bg-default → privateBadgeColourBgDefault

Note: BpkThemeProvider has a digit handler so --bpk-border-1 → key border1.

allBadgeThemeAttributes should be a deduplicated flat array of all private badge vars — not a spread of per-variant arrays (which repeat shared vars). Since normal/warning/success/critical all share privateBadgeColourBgDefault, the combined array should only list it once:

export const allBadgeThemeAttributes = [
  'privateBadgeColourBgDefault',   // normal, warning, success, critical
  'privateBadgeColourBgInverse',   // inverse
  'privateBadgeColourBgOutline',   // outline
  'privateBadgeColourStrokeOutline',
  'privateBadgeColourBgSubtle',    // subtle
];

Step 5: Update stories

Replace old theme keys in BpkThemeProvider story instances:

// Before
<BpkThemeProvider
  theme={{ badgeNormalBackgroundColor: coreAccentDay }}
  themeAttributes={['badgeNormalBackgroundColor']}
>

// After
<BpkThemeProvider
  theme={{ privateBadgeColourBgDefault: coreAccentDay }}
  themeAttributes={['privateBadgeColourBgDefault']}
>

Step 6: Update themeAttributes-test.tsx

Update all .toEqual([...]) assertions to match the new key names.

Verify token choices against Figma

Always cross-check colour tokens against the Figma component. Use get_design_context with disableCodeConnect: true to get the generated code — it includes the CSS var names Figma uses (e.g. --text/primary, --component/badge/colour/bg-default). Strip the slashes and prepend --bpk- to get the SCSS var name.

The Figma-generated code is the source of truth for which colour goes on which variant. Example from badge:

• Normal / Subtle / Inverse / Warning / Success / Critical text: --text/primary → --bpk-text-primary
• Strong / Outline text: --text/on-dark → --bpk-text-on-dark
• Brand text: --text/inverse → --bpk-text-inverse

Verification

# Check no old-style component-specific vars remain as primary values
grep -n "var(--bpk-{component}-" packages/backpack-web/src/bpk-mixins/_{component}.scss

# Run the themeAttributes test
npx jest themeAttributes-test --testPathPattern={component}

Notes

• No nested fallbacks: var(--new-var, sass-fallback). Never var(--old-hook, var(--new-var, sass-fallback)).
• Specificity wins for theming: BpkThemeProvider injects inline styles — no cascade tricks needed.
• --bpk-private-* vars: Component-scoped vars from the theme files. Prefer them for backgrounds. Use global semantic vars (--bpk-text-*, --bpk-core-*) for text and icon colours.
• The outline box-shadow fallback line: Keep a bare SASS token box-shadow declaration before the var() one — for browsers without CSS var support.
• Dimension vars: Component-specific dimension vars (e.g. --bpk-private-{component}-dimension-padding-horizontal-default) take precedence over generic spacing vars where they exist.
• bpk-themeable-property is gone: All colour, spacing, radius and typography properties should use plain var(). The mixin is only still appropriate for legacy theme hooks that have no CSS var equivalent.