diff --git a/src/services/RedocNormalizedOptions.ts b/src/services/RedocNormalizedOptions.ts
index fa730923..777a9175 100644
--- a/src/services/RedocNormalizedOptions.ts
+++ b/src/services/RedocNormalizedOptions.ts
@@ -1,5 +1,5 @@
 import { ThemeInterface } from '../theme';
-import { isNumeric } from '../utils/helpers';
+import { isNumeric, mergeObjects } from '../utils/helpers';
 import defaultTheme from '../theme';
 
 export interface RedocRawOptions {
@@ -33,7 +33,7 @@ export class RedocNormalizedOptions {
   untrustedSpec: boolean;
 
   constructor(raw: RedocRawOptions) {
-    this.theme = { ...(raw.theme || {}), ...defaultTheme }; // todo: merge deep
+    this.theme = mergeObjects({} as any, raw.theme || {}, defaultTheme);
     this.scrollYOffset = RedocNormalizedOptions.normalizeScrollYOffset(raw.scrollYOffset);
     this.hideHostname = RedocNormalizedOptions.normalizeHideHostname(raw.hideHostname);
     this.expandResponses = RedocNormalizedOptions.normalizeExpandResponses(raw.expandResponses);
diff --git a/src/utils/__tests__/helpers.test.ts b/src/utils/__tests__/helpers.test.ts
index 8a18b84a..17428ca5 100644
--- a/src/utils/__tests__/helpers.test.ts
+++ b/src/utils/__tests__/helpers.test.ts
@@ -1,4 +1,4 @@
-import { mapWithLast, appendToMdHeading } from '../helpers';
+import { mapWithLast, appendToMdHeading, mergeObjects } from '../helpers';
 
 describe('Utils', () => {
   describe('helpers', () => {
@@ -38,5 +38,24 @@ describe('Utils', () => {
       const val = appendToMdHeading('', 'Authentication', '<test>');
       expect(val).toEqual('# Authentication\n\n<test>');
     });
+
+    describe('mergeObjects', () => {
+      test('should merge Objects and all nested Ones', () => {
+        const obj1 = { a: { a1: 'A1' }, c: 'C', d: {} };
+        const obj2 = { a: { a2: 'A2' }, b: { b1: 'B1' }, d: null };
+        const obj3 = { a: { a1: 'A1', a2: 'A2' }, b: { b1: 'B1' }, c: 'C', d: null };
+        expect(mergeObjects({}, obj1, obj2)).toEqual(obj3);
+      });
+      test('should behave like Object.assign on the top level', () => {
+        const obj1 = { a: { a1: 'A1' }, c: 'C' };
+        const obj2 = { a: undefined, b: { b1: 'B1' } };
+        expect(mergeObjects({}, obj1, obj2)).toEqual(Object.assign({}, obj1, obj2));
+      });
+      test('should not merge array values, just override', () => {
+        const obj1 = { a: ['A', 'B'] };
+        const obj2 = { a: ['C'], b: ['D'] };
+        expect(mergeObjects({}, obj1, obj2)).toEqual({ a: ['C'], b: ['D'] });
+      });
+    });
   });
 });
diff --git a/src/utils/helpers.ts b/src/utils/helpers.ts
index 8452669f..7b4407a2 100644
--- a/src/utils/helpers.ts
+++ b/src/utils/helpers.ts
@@ -80,3 +80,37 @@ export function appendToMdHeading(md: string, heading: string, content: string)
     return `${md}${br}# ${heading}\n\n${content}`;
   }
 }
+
+// credits https://stackoverflow.com/a/46973278/1749888
+export const mergeObjects = <T extends object = object>(target: T, ...sources: T[]): T => {
+  if (!sources.length) {
+    return target;
+  }
+  const source = sources.shift();
+  if (source === undefined) {
+    return target;
+  }
+
+  if (isMergebleObject(target) && isMergebleObject(source)) {
+    Object.keys(source).forEach(function(key: string) {
+      if (isMergebleObject(source[key])) {
+        if (!target[key]) {
+          target[key] = {};
+        }
+        mergeObjects(target[key], source[key]);
+      } else {
+        target[key] = source[key];
+      }
+    });
+  }
+
+  return mergeObjects(target, ...sources);
+};
+
+const isObject = (item: any): boolean => {
+  return item !== null && typeof item === 'object';
+};
+
+const isMergebleObject = (item): boolean => {
+  return isObject(item) && !Array.isArray(item);
+};