Fix (core): makeTheme should perform deep merges of objects

packages/checkbox/src/index.mts

@@ -103,9 +103,7 @@ export default createPrompt(
       required,
       validate = () => true,
     } = config;
-    const theme = makeTheme<CheckboxTheme>(checkboxTheme, config.theme, {
-      icon: Object.assign({}, checkboxTheme.icon, config.theme?.icon),
-    });
+    const theme = makeTheme<CheckboxTheme>(checkboxTheme, config.theme);
     const prefix = usePrefix({ theme });
     const firstRender = useRef(true);
     const [status, setStatus] = useState('pending');

packages/core/src/lib/make-theme.mts

@@ -1,10 +1,41 @@
 import type { Prettify, PartialDeep } from '@inquirer/type';
 import { defaultTheme, type Theme } from './theme.mjs';
 
+function isPlainObject(value: unknown): value is object {
+  if (typeof value !== 'object' || value === null) return false;
+
+  let proto = value;
+  while (Object.getPrototypeOf(proto) !== null) {
+    proto = Object.getPrototypeOf(proto);
+  }
+
+  return Object.getPrototypeOf(value) === proto;
+}
+
+function deepMerge<T extends object>(...objects: Partial<T>[]): T {
+  const output: { [key: string]: unknown } = {};
+
+  for (const obj of objects) {
+    for (const [key, value] of Object.entries(obj)) {
+      const prevValue = output[key] as unknown;
+
+      if (isPlainObject(prevValue) && isPlainObject(value)) {
+        output[key] = deepMerge(prevValue, value);
+      } else {
+        output[key] = value;
+      }
+    }
+  }
+
+  return output as T;
+}
+
 export function makeTheme<SpecificTheme extends object>(
   ...themes: ReadonlyArray<undefined | PartialDeep<Theme<SpecificTheme>>>
 ): Prettify<Theme<SpecificTheme>> {
-  return Object.assign({}, defaultTheme, ...themes, {
-    style: Object.assign({}, defaultTheme.style, ...themes.map((theme) => theme?.style)),
-  });
+  const themesToMerge = [
+    defaultTheme,
+    ...themes.filter((theme) => theme != null),
+  ] as Theme<SpecificTheme>[];
+  return deepMerge(...themesToMerge);
 }