[DevTools] Rerender when the browser theme changes (facebook#33992)

When the browser theme changes, we don't immediately rerender the UI so
we don't pick up the new theme if the React devtools are set to auto.

This picks up the change immediately.

Author: sebmarkbage

packages/react-devtools-extensions/src/main/index.js

@@ -530,3 +530,16 @@ if (__IS_FIREFOX__) {
 connectExtensionPort();
 
 mountReactDevToolsWhenReactHasLoaded();
+
+function onThemeChanged(themeName) {
+  // Rerender with the new theme
+  render();
+}
+
+if (chrome.devtools.panels.setThemeChangeHandler) {
+  // Chrome
+  chrome.devtools.panels.setThemeChangeHandler(onThemeChanged);
+} else if (chrome.devtools.panels.onThemeChanged) {
+  // Firefox
+  chrome.devtools.panels.onThemeChanged.addListener(onThemeChanged);
+}