diff --git a/src/__tests__/siteImport/cssToStyleRules.test.ts b/src/__tests__/siteImport/cssToStyleRules.test.ts
index e40c54e30..218534731 100644
--- a/src/__tests__/siteImport/cssToStyleRules.test.ts
+++ b/src/__tests__/siteImport/cssToStyleRules.test.ts
@@ -760,3 +760,45 @@ describe('cssToStyleRules — custom @media as conditional layers (no warnings)'
     expect(a.contextStyles[cid]).toMatchObject({ color: 'red', fontSize: '14px' })
   })
 })
+
+// ---------------------------------------------------------------------------
+// Shorthand expansion: `none` must never reach a colour longhand.
+//
+// `background: none` and `border: none` are common resets. Expanding them can
+// put `none` in colour longhands. Browsers reject those declarations, causing
+// an authored reset to stop resetting and expose an underlying theme style.
+// ---------------------------------------------------------------------------
+
+describe('cssToStyleRules — shorthand expansion into colour longhands', () => {
+  it('background: none does not publish background-color: none', () => {
+    const { rules } = cssToStyleRules('.close { background: none }')
+    expect(rules[0].styles.backgroundImage).toBe('none')
+    expect(rules[0].styles.backgroundColor).toBe('initial')
+  })
+
+  it('border: none does not publish border-*-color: none', () => {
+    const { rules } = cssToStyleRules('.close { border: none }')
+    expect(rules[0].styles.borderTopStyle).toBe('none')
+    for (const side of ['borderTopColor', 'borderRightColor', 'borderBottomColor', 'borderLeftColor']) {
+      expect(rules[0].styles[side]).toBe('initial')
+    }
+  })
+
+  it('a real colour is left exactly as authored', () => {
+    const { rules } = cssToStyleRules('.close { background: #e6e6e6; border: 1px solid #ccc }')
+    expect(rules[0].styles.backgroundColor).toBe('#e6e6e6')
+    expect(rules[0].styles.borderTopColor).toBe('#ccc')
+  })
+
+  it('none is still honoured on the properties that accept it', () => {
+    const { rules } = cssToStyleRules('.close { background-image: none; border-style: none }')
+    expect(rules[0].styles.backgroundImage).toBe('none')
+    expect(rules[0].styles.borderTopStyle).toBe('none')
+  })
+
+  it('none survives verbatim in camelCase and kebab-case custom properties', () => {
+    const { rules } = cssToStyleRules('.card { --myColor: none; --my-color: none }')
+    expect(rules[0].styles['--myColor']).toBe('none')
+    expect(rules[0].styles['--my-color']).toBe('none')
+  })
+})
diff --git a/src/core/css-substitution/index.ts b/src/core/css-substitution/index.ts
index 5f6e10410..e9bfd0953 100644
--- a/src/core/css-substitution/index.ts
+++ b/src/core/css-substitution/index.ts
@@ -230,6 +230,25 @@ export interface CssDeclarationBlock {
   priorities: CSSDeclarationPriorityBag
 }
 
+/**
+ * Repair a longhand expanded from a shorthand.
+ *
+ * `background: none` and `border: none` are everyday CSS, but expansion can
+ * put `none` into colour longhands such as `background-color` and
+ * `border-top-color`. No colour property accepts `none`, so the browser drops
+ * that declaration and the reset stops resetting.
+ *
+ * `initial` supplies the intended initial value in both cases: `transparent`
+ * for a background and `currentcolor` for a border.
+ * This intentionally also repairs an author-typed invalid colour longhand;
+ * otherwise the browser would drop it and could preserve an earlier value.
+ */
+function repairExpandedShorthandValue(camel: string, value: string): string {
+  if (camel.startsWith('--')) return value
+  if (!camel.endsWith('Color')) return value
+  return value.trim().toLowerCase() === 'none' ? 'initial' : value
+}
+
 /**
  * Walk a parsed `CSSStyleDeclaration` into a camelCase property bag:
  * decodes substitution markers back to their real property
@@ -262,7 +281,7 @@ export function readCssDeclarationBlock(
       onBlockedProperty?.(camel, kebab)
       continue
     }
-    styles[camel] = value
+    styles[camel] = repairExpandedShorthandValue(camel, value)
     if (style.getPropertyPriority(rawKebab).toLowerCase() === 'important') {
       priorities[camel] = 'important'
     }