Add annotations or make things readonly to prepare for object literal soundness fix in xplat/js: 1/n (#51912)

Summary:
Pull Request resolved: #51912

Changelog: [Internal]

Reviewed By: marcoww6

Differential Revision: D76292113

fbshipit-source-id: facaf3e503b69ba41df7842b0e92b7094c271874

Author: SamChou19815

packages/rn-tester/js/components/RNTesterTheme.js

@@ -15,7 +15,7 @@ import {createContext} from 'react';
 import {use} from 'react';
 import {Appearance} from 'react-native';
 
-export type RNTesterTheme = {
+export type RNTesterTheme = $ReadOnly<{
   LabelColor: ColorValue,
   SecondaryLabelColor: ColorValue,
   TertiaryLabelColor: ColorValue,
@@ -54,7 +54,7 @@ export type RNTesterTheme = {
   NavBarPlaygroundActiveIcon: ImageSource,
   NavBarPlaygroundInactiveIcon: ImageSource,
   ...
-};
+}>;
 
 export const RNTesterLightTheme = {
   LabelColor: '#000000ff',