diff --git a/apps/storybook/stories/Avatar.stories.tsx b/apps/storybook/stories/Avatar.stories.tsx
index 2c4768ef28cf..7bbc1168138e 100644
--- a/apps/storybook/stories/Avatar.stories.tsx
+++ b/apps/storybook/stories/Avatar.stories.tsx
@@ -4,6 +4,7 @@ import type {Meta, StoryObj} from '@storybook/react';
 import * as stylex from '@stylexjs/stylex';
 import {Avatar} from '@astryxdesign/core/Avatar';
 import {AvatarStatusDot} from '@astryxdesign/core/Avatar';
+import {Theme, defineTheme} from '@astryxdesign/core/theme';
 import {
   spacingVars,
   typographyVars,
@@ -555,3 +556,55 @@ export const NumericSizes: Story = {
     </div>
   ),
 };
+
+// A theme can re-scope the fallback initials' typography per size tier via the
+// Avatar-scoped derived vars — a smaller-per-size type scale, regular weight,
+// and a muted secondary-text color on an accent wash fill — without forking the
+// component. The default row is unchanged (size × 0.4, medium weight, neutral
+// fill); only the themed row opts in.
+const fallbackScaleTheme = defineTheme({
+  name: 'avatar-fallback-scale',
+  components: {
+    avatar: {
+      base: {
+        fontWeight: 'var(--font-weight-normal)',
+        color: 'var(--color-text-secondary)',
+        backgroundColor: 'var(--color-accent-muted)',
+      },
+      'size:xsm': {fontSize: '8px'},
+      'size:sm': {fontSize: '9px'},
+      'size:md': {fontSize: '13px'},
+      'size:lg': {fontSize: '16px'},
+      'size:xl': {fontSize: '40px'},
+    },
+  },
+});
+
+export const ThemedFallbackScale: Story = {
+  name: 'Themed Fallback Type Scale',
+  render: () => (
+    <div {...stylex.props(styles.storyWrapper)}>
+      <h4 {...stylex.props(styles.heading)}>Default fallback (size × 0.4)</h4>
+      <div {...stylex.props(styles.row)}>
+        <Avatar name="TY" size="xsm" />
+        <Avatar name="XS" size="sm" />
+        <Avatar name="SM" size="md" />
+        <Avatar name="MD" size="lg" />
+        <Avatar name="LG" size="xl" />
+      </div>
+
+      <h4 {...stylex.props(styles.heading)}>
+        Themed fallback (per-size scale, regular weight, wash fill)
+      </h4>
+      <Theme theme={fallbackScaleTheme} mode="light">
+        <div {...stylex.props(styles.row)}>
+          <Avatar name="TY" size="xsm" />
+          <Avatar name="XS" size="sm" />
+          <Avatar name="SM" size="md" />
+          <Avatar name="MD" size="lg" />
+          <Avatar name="LG" size="xl" />
+        </div>
+      </Theme>
+    </div>
+  ),
+};
diff --git a/packages/core/src/Avatar/Avatar.doc.mjs b/packages/core/src/Avatar/Avatar.doc.mjs
index 89f0d2a73d4d..d39577e124f6 100644
--- a/packages/core/src/Avatar/Avatar.doc.mjs
+++ b/packages/core/src/Avatar/Avatar.doc.mjs
@@ -30,6 +30,18 @@ export const docs = {
       {className: 'astryx-avatar', visualProps: ['size']},
       {className: 'astryx-avatar-status-dot', visualProps: ['variant']},
     ],
+    vars: [
+      {name: '--_avatar-fallback-font-size', description: 'Initials font size; default is proportional to the avatar size (size × 0.4). Override per size tier (e.g. size:sm) for a custom type scale.', default: 'calc(avatar-size × 0.4)', private: true},
+      {name: '--_avatar-fallback-font-weight', description: 'Initials font weight', default: 'var(--font-weight-medium)', private: true},
+      {name: '--_avatar-fallback-color', description: 'Fallback text and default-icon color', default: 'var(--color-text-secondary)', private: true},
+      {name: '--_avatar-fallback-background', description: 'Fallback wash background fill', default: 'var(--color-neutral)', private: true},
+    ],
+    derived: [
+      {property: 'fontSize', vars: ['--_avatar-fallback-font-size']},
+      {property: 'fontWeight', vars: ['--_avatar-fallback-font-weight']},
+      {property: 'color', vars: ['--_avatar-fallback-color']},
+      {property: 'backgroundColor', vars: ['--_avatar-fallback-background']},
+    ],
   },
   description: 'Displays a user avatar with image, initials fallback, and optional status indicator.',
   props: [
diff --git a/packages/core/src/Avatar/Avatar.test.tsx b/packages/core/src/Avatar/Avatar.test.tsx
index aced7382915d..513c600603e5 100644
--- a/packages/core/src/Avatar/Avatar.test.tsx
+++ b/packages/core/src/Avatar/Avatar.test.tsx
@@ -35,6 +35,19 @@ describe('Avatar', () => {
     expect(innerImg).toHaveAttribute('alt', '');
   });
 
+  it('renders fallback initials through the themeable font-size var, not a bare px literal', () => {
+    render(<Avatar name="Ada Lovelace" size="sm" data-testid="a" />);
+    const initials = screen.getByText('AL');
+    // The seam: the dynamic font size resolves to the Avatar-scoped var (with
+    // the proportional `size × 0.4` default baked in as the fallback), so a
+    // theme can re-scope it per size. A regression to a bare px literal would
+    // break theming.
+    const style = initials.getAttribute('style') ?? '';
+    expect(style).toContain('var(--_avatar-fallback-font-size,');
+    // Default still reproduces the proportional scale (sm = 24 × 0.4 = 9.6px).
+    expect(style).toMatch(/var\(--_avatar-fallback-font-size,\s*9\.6\d*px\)/);
+  });
+
   it('retries a new src after a previous src failed to load', () => {
     const {rerender} = render(
       <Avatar name="Ada" src="https://example.com/broken.jpg" />,
diff --git a/packages/core/src/Avatar/Avatar.tsx b/packages/core/src/Avatar/Avatar.tsx
index 2db1ef9493a8..d58f15e060fd 100644
--- a/packages/core/src/Avatar/Avatar.tsx
+++ b/packages/core/src/Avatar/Avatar.tsx
@@ -104,6 +104,11 @@ const styles = stylex.create({
     position: 'relative',
     display: 'inline-flex',
     flexShrink: 0,
+    // The wrapper is not clipped (so the status dot can overflow), so it must be
+    // rounded itself: a themed fallback background lands on `.astryx-avatar` (the
+    // class-bearing wrapper) as well as the internal var, and an unrounded
+    // wrapper would show that fill as square corners behind the circular content.
+    borderRadius: radiusVars['--radius-full'],
   },
   content: {
     display: 'flex',
@@ -124,10 +129,14 @@ const styles = stylex.create({
     justifyContent: 'center',
     width: '100%',
     height: '100%',
-    backgroundColor: colorVars['--color-neutral'],
-    color: colorVars['--color-text-secondary'],
+    // Fallback surface (initials + default icon). Each property reads an
+    // Avatar-scoped internal var so a theme can re-scope the fallback wash and
+    // initials weight/color without forking; the defaults reproduce today's
+    // exact output. See derivedVarRegistry (avatar) + Avatar.doc.mjs theming.
+    backgroundColor: `var(--_avatar-fallback-background, ${colorVars['--color-neutral']})`,
+    color: `var(--_avatar-fallback-color, ${colorVars['--color-text-secondary']})`,
     fontFamily: typographyVars['--font-family-body'],
-    fontWeight: fontWeightVars['--font-weight-medium'],
+    fontWeight: `var(--_avatar-fallback-font-weight, ${fontWeightVars['--font-weight-medium']})`,
     textTransform: 'uppercase',
   },
   status: {
@@ -143,8 +152,13 @@ const dynamicStyles = stylex.create({
     width: size,
     height: size,
   }),
+  // Initials font size defaults to the proportional `size × ratio` scale but is
+  // reachable via the `--_avatar-fallback-font-size` derived var, so a theme can
+  // set a per-size type scale (e.g. `components.avatar['size:sm']`).
   fontSize: (size: number) => ({
-    fontSize: size * INITIALS_FONT_SIZE_RATIO,
+    fontSize: `var(--_avatar-fallback-font-size, ${
+      size * INITIALS_FONT_SIZE_RATIO
+    }px)`,
   }),
   statusPosition: (size: number) => ({
     bottom: size * CIRCLE_EDGE_OFFSET_RATIO,
diff --git a/packages/core/src/theme/derivedVarRegistry.test.ts b/packages/core/src/theme/derivedVarRegistry.test.ts
index 94b52e14a301..61d3a3d99299 100644
--- a/packages/core/src/theme/derivedVarRegistry.test.ts
+++ b/packages/core/src/theme/derivedVarRegistry.test.ts
@@ -186,6 +186,7 @@ function discoverComponents(): ComponentInfo[] {
 // ---------------------------------------------------------------------------
 
 const DIR_TO_REGISTRY_KEY: Record<string, string> = {
+  Avatar: 'avatar',
   Banner: 'banner',
   Button: 'button',
   Card: 'card',
diff --git a/packages/core/src/theme/derivedVarRegistry.ts b/packages/core/src/theme/derivedVarRegistry.ts
index 3a037c2f4a03..88449e8ee613 100644
--- a/packages/core/src/theme/derivedVarRegistry.ts
+++ b/packages/core/src/theme/derivedVarRegistry.ts
@@ -35,6 +35,12 @@ export interface DerivedVarEntry {
  * entries share the same property.
  */
 export const derivedVarRegistry: Record<string, DerivedVarEntry[]> = {
+  avatar: [
+    {property: 'fontSize', vars: ['--_avatar-fallback-font-size']},
+    {property: 'fontWeight', vars: ['--_avatar-fallback-font-weight']},
+    {property: 'color', vars: ['--_avatar-fallback-color']},
+    {property: 'backgroundColor', vars: ['--_avatar-fallback-background']},
+  ],
   banner: [{property: 'borderRadius', vars: ['--_banner-radius']}],
   button: [{property: 'borderRadius', vars: ['--_button-radius']}],
   card: [
diff --git a/packages/core/src/theme/generateThemeRules.test.ts b/packages/core/src/theme/generateThemeRules.test.ts
index b77d671143df..df61188b1875 100644
--- a/packages/core/src/theme/generateThemeRules.test.ts
+++ b/packages/core/src/theme/generateThemeRules.test.ts
@@ -385,6 +385,48 @@ describe('derived var expansion', () => {
     expect(rule).toContain('--_button-radius: 8px');
   });
 
+  it('emits internal vars for avatar fallback typography (base)', () => {
+    const theme = defineTheme({
+      name: 'test-derived-avatar',
+      components: {
+        avatar: {
+          base: {
+            fontWeight: 'var(--font-weight-normal)',
+            color: 'var(--color-text-secondary)',
+            backgroundColor: 'var(--color-accent-muted)',
+          },
+        },
+      },
+    });
+    const rules = generateThemeRules(theme);
+    const rule = rules.find(r => r.includes('.astryx-avatar'));
+    expect(rule).toBeDefined();
+    expect(rule).toContain(
+      '--_avatar-fallback-font-weight: var(--font-weight-normal)',
+    );
+    expect(rule).toContain(
+      '--_avatar-fallback-color: var(--color-text-secondary)',
+    );
+    expect(rule).toContain(
+      '--_avatar-fallback-background: var(--color-accent-muted)',
+    );
+  });
+
+  it('emits a per-size fallback font-size for avatar (size:sm)', () => {
+    const theme = defineTheme({
+      name: 'test-derived-avatar-size',
+      components: {
+        avatar: {
+          'size:sm': {fontSize: '9px'},
+        },
+      },
+    });
+    const rules = generateThemeRules(theme);
+    const rule = rules.find(r => r.includes('.astryx-avatar.sm'));
+    expect(rule).toBeDefined();
+    expect(rule).toContain('--_avatar-fallback-font-size: 9px');
+  });
+
   it('does not emit derived vars for components without registry entries', () => {
     const theme = defineTheme({
       name: 'test-no-derived',