When you edit a page this top bar is auto generated. If you want to add background there you need to add this as visual composer element.

1. Edit Page

2. Hide bar by changing page template to “Full Width”

3. Click the Update button

4. Edit with Frontend editor

4. Add new row at the top of the site

5. Change settings

• Row stretch => Strech row

• Padding top and bottom: 48px.
• Choose your background image
• Change background to: Cover

5. Save

6. Add Heading Element inside the row