Adding to unreleased

Author: Dominic McPhee

UNRELEASED.md

@@ -10,6 +10,8 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 
 Updated `Textfield` with a `type` of `number` to not render a spinner if step is set to `0` ([#3477](https://github.com/Shopify/polaris-react/pull/3477))
 
+- Optimized `ThemeProvider` to only output its custom properties in nested `ThemeProvider`s when they differ from the parent context ([#3550](https://github.com/Shopify/polaris-react/pull/3550))
+
 ### Bug fixes
 
 Fixed `Filters` overflow ([#3532](https://github.com/Shopify/polaris-react/pull/3532))

src/components/ThemeProvider/ThemeProvider.tsx

@@ -101,8 +101,7 @@ export function ThemeProvider({
     !isParentThemeProvider &&
     parentContext!.cssCustomProperties !== toString(customProperties)
   ) {
-    customProperties.color = color;
-    style = customProperties;
+    style = {...customProperties, ...{color}};
   } else {
     style = {color};
   }