message: feat(extend-theme): do not mutate theme and specify return type explicitly
diff: @@ -13,7 +13,7 @@ export function extendTheme<T extends Theme | Record<string, any>>(
  ? overrideThunk(...args)
  : overrideThunk
  - return mergeWith(sourceValue, overrideValue, customizer)
  + return mergeWith({}, sourceValue, overrideValue, customizer)
  }
  }
  @@ -21,5 +21,5 @@ export function extendTheme<T extends Theme | Record<string, any>>(
  return undefined
  }
  - return mergeWith(defaultTheme, overrides, customizer)
  + return mergeWith({}, defaultTheme, overrides, customizer)
  }
  